What will happen if the Custom GL Plug-in implementation (Customization > Plug-ins > Manage Plug-in Implementation) is disabled?
On View of Existing Record, custom lines are still present.
On Edit and Save of Existing Record, custom lines will be removed.
On New Record, custom lines will not be displayed.
What will happen to the transaction with existing custom GL Lines when the Plug-in implementation script is set to inactive?
The custom lines will be removed, same as if you would disable the Plug-in implementation.
Is server-side/client script/workflow going to execute first before the custom GL Plugin?
It depends on the script deployment. The help documentation contains an explanation of the Custom GL Lines flow but it executes between Before Submit and After Submit.
Does Custom GL Plugin have any impact on the script attached to the transaction record?
Only if the script is deployed After Submit and is related to the transaction GL Lines. In general, there is no impact.
Can users modify or replace standard GL lines?
Users can only add custom GL lines to the standard GL Impact of transactions. It is not possible to modify or replace standard GL lines.