ST_SRID — Returns the spatial reference identifier for a topogeometry.
integer ST_SRID(topogeometry tg
Returns the spatial reference identifier for the ST_Geometry as defined in spatial_ref_sys table. Section 4.5, “Spatial Reference Systems”
spatial_ref_sys table is a table that catalogs all spatial reference systems known to PostGIS and is used for transformations from one spatial reference system to another. So verifying you have the right spatial reference system identifier is important if you plan to ever transform your geometries.
This method implements the SQL/MM specification. SQL-MM 3: 14.1.5