Reference:
https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/data-entities/entity-change-track

One use of the SysObsoleteAttribute class is to notify the compiler that the compile should fail if a particular method is called in the source code. The compiler rejects the compile, and displays the specific message that is stored in this use of the attribute.
Reference:
https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/dev-ref/xpp-attribute-classes

String size can be defined only on root EDTs. The system will use the largest value that is defined across the EDT and its extensions.
Labels and Help text properties can be changed by an extension, but only one value can remain.
Reference:
https://docs.microsoft.com/en-us/dynamics365/fin-ops-core/dev-itpro/extensibility/extensible-edts

Explanation:
Step 1: Copy the validatingWrite event handler on the CustTable object (it is either validatingWrite or validatedWrite, there is no such option as validateWrite).
Step 2: Add the event handler method to a class.
Step 3: Add code to perform the validation.