The Siemens 6ES7951-1KM00-0AA0 is a spare-part memory card for the S7-300 PLC family. It provides 4 Mbyte of Flash-EPROM storage, operating at 5V, in a short-design form factor. This is the memory that holds the user program and data for the CPU — without it, or with a full one, the controller won't load its application.
What the key rating means for fit
The 4 Mbyte memory size determines how large a program and data block the S7-300 CPU can load. For a typical medium-size machine with a few hundred I/O points and moderate data logging, 4 Mbyte is sufficient. If the existing program exceeds this, the card won't fit — check the CPU's memory usage in TIA Portal or Step 7 before ordering.