knowledge(data-modeling): move Prices Including VAT article to Microsoft layer

data-modeling is a Microsoft-owned review domain consumed by
microsoft/skills/review/al-data-modeling-review.md, and
docs/contributing.md does not allow Community as a staging layer for
such domains. Move the article and its .good.al/.bad.al samples to
microsoft/knowledge/data-modeling/. Content is unchanged; the slug-based
evaluation entry stays as is.

microsoft/knowledge/data-modeling/document-line-prices-follow-prices-including-vat.md

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+---
+bc-version: [all]
+domain: data-modeling
+keywords: [prices-including-vat, unit-price, line-amount, direct-unit-cost, prepmt-line-amount, amount-including-vat, sales-line, purchase-line, service-line, net-gross]
+technologies: [al]
+countries: [w1]
+application-area: [all]
+---
+
+# Sales, purchase, and service line prices follow the header's Prices Including VAT
+
+> Contributions welcome — open a PR to refine or extend this article.
+
+## Description
+
+Line prices are gross or net depending on the header's `Prices Including VAT`. When the flag is set, `Unit Price` (sales, service), `Direct Unit Cost` (purchase), `Line Amount`, `Line Discount Amount`, `Inv. Discount Amount`, and the prepayment fields `Prepmt. Line Amount`, `Prepmt. Amt. Inv.`, `Prepmt Amt to Deduct`, and `Prepmt Amt Deducted` all include VAT. When it is cleared they exclude it. Only `Amount`, `VAT Base Amount`, and `Prepayment Amount` (always net) and `Amount Including VAT`, `Prepmt. Amt. Incl. VAT`, and `Prepmt. Amount Inv. Incl. VAT` (always gross) keep a fixed basis. Code that mixes the two groups without looking at the header flag is wrong for every document whose customer or vendor uses the other setting.
+
+## Best Practice
+
+When code needs a known basis — exports, integrations, KPIs, custom totals, commission or margin calculations — read `Amount` for net and `Amount Including VAT` for gross. Both are already reduced by line and invoice discounts and are maintained by the line's VAT calculation, so no VAT arithmetic is needed.
+
+When code combines fields, pair fields of the same group. BaseApp's `UpdatePrepmtAmounts` sets `Prepmt. Line Amount` from `Line Amount` minus `Inv. Discount Amount`. This is correct because all three follow the header flag. `Prepmt. Amt. Inv.` pairs with `Prepmt. Line Amount`; `Prepmt. Amount Inv. Incl. VAT` pairs only with other gross values.
+
+When code writes a price from an external source whose basis is known (an EDI price list, an API payload, a web-shop order), get the document header and convert the source price into the header's basis before validating `Unit Price` or `Direct Unit Cost`, using the line's `VAT %` and the currency's `Unit-Amount Rounding Precision`. Alternatively, set `Prices Including VAT` on the header to match the source before the first line is created. Toggling it later on a sales header with priced lines asks the user to confirm a recalculation. Without a UI session, or when validation dialogs are hidden, BaseApp converts all line prices without asking.
+
+The standard price calculation already converts a `Price List Line` whose `Price Includes VAT` differs from the document's setting, so a price it returns is in the document's basis and must not be converted a second time.
+
+On purchase lines, `Unit Cost` and `Unit Cost (LCY)` are derived from `Direct Unit Cost` with VAT removed, so they stay net. Service lines follow the same rule for `Unit Price` and `Line Amount`: they share the caption switch and the `UpdateVATAmounts` split of the sales line.
+
+See sample: [`document-line-prices-follow-prices-including-vat.good.al`](document-line-prices-follow-prices-including-vat.good.al).
+
+## Anti Pattern
+
+Treating `Unit Price`, `Direct Unit Cost`, or `Line Amount` as net by default. Typical signals: summing `Line Amount` as a document's net total, computing VAT as `Line Amount * "VAT %" / 100`, putting a known net or gross external price straight into `Unit Price` or `Direct Unit Cost`, or comparing a line price with `Item."Unit Price"` or `Item."Last Direct Cost"` — all without reading the header's `Prices Including VAT`. For a gross-price customer at 19 % VAT, a net total built from `Line Amount` is 19 % too high, and a net price of 100 imported unconverted yields a net revenue of only 84.03.
+
+Trusting a name instead of the source fields. The public procedure `CalculateOutstandingAmountExclTax` on `Sales Line` and `Purchase Line` returns `Line Amount` minus `Inv. Discount Amount` for the uninvoiced quantity, so its result includes VAT on a `Prices Including VAT` document despite its name. BaseApp only combines it with `Prepmt. Line Amount`, which has the same basis. Extension code that uses it as a net outstanding amount — compared with `Amount`, exported as net, or used to compute VAT — has this defect. The same applies to any variable or procedure named `ExclVAT`, `ExclTax`, or `Net` that is fed from a header-dependent field.
+
+Do not report code that reads the header flag, uses only fixed-basis fields, or only combines fields of the same group (for example, prepayment amounts derived from `Line Amount`, or values copied between two lines of the same document).
+
+See sample: [`document-line-prices-follow-prices-including-vat.bad.al`](document-line-prices-follow-prices-including-vat.bad.al).
+
+## References
+
+- [BCApps: Sales Header `Prices Including VAT` OnValidate recalculates line prices](https://github.com/microsoft/BCApps/blob/main/src/Layers/W1/BaseApp/Sales/Document/SalesHeader.Table.al).
+- [BCApps: Sales Line `UpdateVATAmounts`, `UpdatePrepmtAmounts`, and `CalculateOutstandingAmountExclTax`](https://github.com/microsoft/BCApps/blob/main/src/Layers/W1/BaseApp/Sales/Document/SalesLine.Table.al).
+- [BCApps: `Sales Line CaptionClass Mgmt` switches captions to Incl./Excl. VAT](https://github.com/microsoft/BCApps/blob/main/src/Layers/W1/BaseApp/Sales/Document/SalesLineCaptionClassMgmt.Codeunit.al).
+- [BCApps: Purchase Line `UpdateUnitCost` removes VAT from `Direct Unit Cost`](https://github.com/microsoft/BCApps/blob/main/src/Layers/W1/BaseApp/Purchases/Document/PurchaseLine.Table.al).
+- [BCApps: Service Line `GetCaptionClass` and `UpdateVATAmounts`](https://github.com/microsoft/BCApps/blob/main/src/Layers/W1/BaseApp/Service/Document/ServiceLine.Table.al).
+- [BCApps: `Price Calculation Buffer Mgt.` `ConvertAmountByTax`](https://github.com/microsoft/BCApps/blob/main/src/Layers/W1/BaseApp/Pricing/Calculation/PriceCalculationBufferMgt.Codeunit.al).