The copying behaviour does not happen for customer, where I have to actually call the setEntityId() function. The copying does not happen for memo either. Maybe it only copies classifications? I'm sure it wasn't doing this until I added code to set the customer/entity, and now those classification fields are being copied over from the standard GL lines to the custom GL lines. Why the inconsistency? Why the lack of documentation on this behaviour? You really want well-documented, predictable, and consistent behaviour when you're modifying the GL.

"The default values for column segments are sourced from the body" Custom GL Lines Plug-in Overview