Hello bishoui;

In STL you can setup a grouping of instructions that you want to call at a later point in your program, either conditionnally or unconditionally. To identify these grouping of instructions, you use a Label.

In your case m02b: is a label that you point to with a Jump instruction. An STL label is 4 characters long and must start with a letter.

The screenshot below explains it better than I can, it is taken from the STL for S7-300/400 manual:

https://support.industry.siemens.com/cs/ww/en/view/109751814

Hope this helps,

Daniel Chartier