There is no external documentation available for the _t_* tables.  In general documentation on the system catalog tables and views is very sparse even where it is available.

The nzsql command has an option -E  which causes the SQL for the internal  backslash commands to be echoed prior to the commands running.  This provides insight into how things work internally.    Like most vendors Netezza makes a point of stating that system catalogs change without notice. The views _v_*  change less, so if your better off using a view if your writting code and want to avoid change from release to release.

Views which start with _v_odbc, _v_jdbc, _v_oledb , ... are typically tied to the standard and tend to be very stable, so if you want information which is published via an interface standard getting it from these view will likely result in few visits to fix the view from release to release.