The Siemens 6ES7953-8LF00-0AA0 is a Micro Memory Card for the S7-300, C7, and ET 200S IM151 CPU families. It stores the program and configuration data for the CPU, using a 64 kbyte Flash-EPROM that retains the data without a backup battery.
64 kbyte is the available user memory for logic blocks, data blocks, and system configuration. For a typical small-to-medium S7-300 application — say, a conveyor line with a few dozen I/O points and basic interlocks — this is enough. If your project uses extensive function blocks, recipe data, or a large HMI tag database, you will hit the ceiling. The Flash-EPROM type means the card is electrically erased and reprogrammed in-circuit; no UV eraser needed, and no battery to maintain the program during a power loss.