Parts on a API page should have PageType = API

[Arthurvdv]

In this example below the part uses a page which isn't optimized of API handling.

page 50100 MyApiPage
{
    PageType = API;
    APIPublisher = 'myPublisher';
    APIGroup = 'myGroup';
    APIVersion = 'v2.0';
    EntityName = 'salesOrder';
    EntitySetName = 'salesOrders';
    ODataKeyFields = SystemId;
    SourceTable = "Sales Header";
    DelayedInsert = true;
    SourceTableView = where("Document Type" = const(Order));

    layout
    {
        area(Content)
        {
            repeater(General)
            {
                field(id; Rec.SystemId) { }
                part(salesOrderLines; "Sales Order Subform") // Raise a rule that the part isn't of PageType API
                {
                    Caption = 'Lines';
                    EntityName = 'salesOrderLine';
                    EntitySetName = 'salesOrderLines';
                    SubPageLink = "Document Type" = field("Document Type"), "Document No." = field("No.");
                }
            }
        }
    }
}