Hi AJ
for stacks to match its always best to have the exact same ios in place to get all features
if that's not possible Cisco uses the major and minor numbers , if they fully match you will get all features , if they partially match you will only get some. 12.4(15) would be an example of the major minor numbers from and IOS code , this doc explains more
http://www.cisco.com/c/en/us/products/collateral/ios-nx-os-software/ios-software-releases-12-4-mainline/whitepaper_C11-719867.html
Major Version Number Incompatibility Among Switches
Switches with different major Cisco IOS software versions usually have different stack protocol versions. Switches with different major version numbers are incompatible and cannot exist in the same switch stack.
Minor Version Number Incompatibility Among Switches
Switches with the same major version number but with a different minor version number are considered partially compatible. When connected to a switch stack, a partially compatible switch enters version-mismatch (VM) mode and cannot join the stack as a fully functioning member. The software detects the mismatched software and tries to upgrade (or downgrade) the switch in VM mode with the switch stack image or with a tar file image from the switch stack flash memory. The software uses the automatic upgrade (auto-upgrade) and the automatic advise (auto-advise) features. For more information, see the "Understanding Auto-Upgrade and Auto-Advise" section.
To see if there are switches in VM mode, use the show switch user EXEC command. The port LEDs on switches in VM mode stay off. Pressing the Mode button does not change the LED mode.
You can use the boot auto-download-sw global configuration command to specify a URL pathname for the master switch to use to get an image in case of version mismatch.
http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-2_55_se/configuration/guide/3750xscg/swstack.html#wp1223384
