The Siemens 6ES7951-0KJ00-0AA0 is a spare-part memory card for the SIMATIC S7-300 PLC family. It stores the user program and configuration data in 512 kbyte of Flash-EPROM, which retains the content without backup battery. The short-design form factor fits the standard S7-300 CPU slot.
What the key rating means for fit
The 512 kbyte capacity is the program memory size available to the S7-300 CPU. It determines how large a control program (blocks, DBs, FCs, FBs) the card can hold. For a typical medium-sized line with a few hundred I/O points and moderate logic, 512 kbyte is adequate; for a large multi-axis motion or recipe-heavy application, you would need the 2 Mbyte or 8 Mbyte card. The Flash-EPROM type means the card can be erased and reprogrammed in the field using the programming software — no UV eraser needed.