![[Pasted image 20231130161926.png]]

the "keys" at the backside of the module avoid that you can plug in a "wrong" module.

e.g. you have a DI-module; the key allows only to plug in a DI-module and not a DQ or AI  or...

For the most modules you have the same type of base-unit.  
If you plug in your modules at the first time then you have a pattern at the base-unit.  
If you want to change the module you can't plug the wrong part.

https://support.industry.siemens.com/forum/it/en/posts/et200sp-io-module-installation/252172