How is the grid table identifier calculated

This article details out how the grid table identifier (table name) is actually constructed.

The table name is generated by the addon using the base name and the context id so if you have in the gridconfiguration

gd.tablename = invoice

then the table in the database will be called
invoice_c12345

Where 12345 is the unique context id of the customfield.
Check connecting the grid to a database for more details

Table Grid Editor (legacy) Documentation