The NAS driver reads the NAS/ALKIS format used for cadastral data in Germany. The format is a GML profile with fairly complex GML3 objects not easily read with the general OGR GML driver.
This driver depends on GDAL/OGR being built with the Xerces XML parsing library.
The driver looks for "opengis.net/gml" and one of the strings semicolon separated strings listed in the option NAS_INDICATOR (which defaults to "NAS-Operationen;AAA-Fachschema;aaa.xsd;aaa-suite") to determine if a input is a NAS file and ignores all files without any matches.
In GDAL 2.3 a bunch of workarounds were removed, that caused the driver to remap or ignore some elements and attributes internally to avoid attribute conflicts (e.g. zeigtAufExternes). Instead it now takes the NAS_GFS_TEMPLATE option, that makes it possible to cleanly map element paths to feature attributes using a GFS file like in the GML driver. Multiple geometries per layer are also possible (eg. ax_flurstueck.objektkoordinaten next to the regular wkb_geometry).
A GFS template and a corresponding PostgreSQL schema of the full NAS schema are part of norGIS ALKIS-Import (also featuring a shell script and PyQt frontend which ease the import). The two files were generated using xmi2db (fork of xmi2db) from the official application schema.
New in 2.3 is also the option NAS_NO_RELATION_LAYER that allows to disable populating the table alkis_beziehungen. The information found there is redundant to the relation fields also contained in original elements/tables. Enabling the option also makes progress reporting available.
This driver was implemented within the context of the PostNAS project, which has more information on its use and other related projects.