These letters tell you which Feature set is running on the switch. If you have a WS-C2960-24PC-S it's running the LAN Lite software, and the WS-C2960-24-PC-L is running the LAN Base software with more features.
You can find this info in the Data-Sheets of the particular product:
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...