The 6ES7953-8LJ11-0AA0: The 512 KB capacity is the key decision point. For a typical S7-300 application, this holds a medium-sized logic program with a few data blocks — think a small conveyor line or a pump skid. If your project runs a larger CPU like a 315-2DP with extensive function blocks and PID loops, you'll want to step up to a 2 MB or 4 MB card. The 3.3 V Nflash interface is standard for these CPUs, so no compatibility surprises there.
This card lives inside the CPU slot of an S7-300 rack, a C7 compact unit, or an ET 200S IM151 interface module. It's the program carrier for those controllers, which are common in mid-range automation for material handling, packaging, water treatment, and machine tools. The card itself is swapped in seconds — power down, pop the old one out, slide the new one in, power up and download.