Spatial Reference Identifiers (SRIDs)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Each spatial instance has a spatial reference identifier (SRID). The SRID corresponds to a spatial reference system based on the specific ellipsoid used for either flat-earth mapping or round-earth mapping.
A spatial column can contain objects with different SRIDs. However, only spatial instances with the same SRID can be used when performing operations with SQL Server spatial data methods on your data. The result of any spatial method derived from two spatial data instances is valid only if those instances have the same SRID that is based on the same unit of measurement, datum, and projection used to determine the coordinates of the instances. The most common units of measurement of a SRID are meters or square meters.
If two spatial instances do not have the same SRID, the results from a geometry or geography Data Type method used on the instances will return NULL. For example, for the following predicate term to return a non-NULL result, the two geometry instances, geometry1 and geometry2, must have the same SRID:
geometry1.STIntersects(geometry2) = 1
Note
The spatial reference identification system is defined by the European Petroleum Survey Group (EPSG) standard, which is a set of standards developed for cartography, surveying, and geodetic data storage. This standard is owned by the Oil and Gas Producers (OGP) Surveying and Positioning Committee.