NEMA (National Electrical Manufacturers Assoc) publishes standard frame sizes for motors up to around 350HP (depending on several factors). Within those designated frame sizes, specific critical dimensions are supposed to be the same regardless of manufacturer.
https://www.engineeringtoolbox.com/nema-electrical-motor-frame-dimensions-d_1504.html
Above that, motors are referred to as "Above NEMA" and frame sizes mean nothing to anyone outside of a specific manufacturer. So in this case, a 509S is a designation for a Siemens "Above NEMA" motor frame size, probably Medium Voltage. Other mfrs use a "509" frame designation in their MV motors as well, but there is no official standard any longer so you can't count on anything. That's because on those large motors, they are typically made to order, so you can ask for whatever you need anyway.
https://www.industry.usa.siemens.co...cification/Documents/CZ-500-2plusPL-AF-FS.pdf