Given a custom column named u_custom_column on table cmdb_ci_linux_server, which variable syntax should be used...

The correct variable syntax to populate a custom column in a horizontal discovery pattern is $cmdb_ci_linux_server.u_custom_column.INSERT. This syntax indicates that the value of the custom column will be inserted into the cmdb_ci_linux_server table for the current CI. The other options are either invalid or incorrect syntaxes for this operation12.

References:

Set Parameter Value operation

Pattern variables