The possible breakpoint positions depend on the editor. Basically they are those positions in a POU where values of variables can change or where the program flow branches out or another POU is called.
See in the following, the various symbols for indicating breakpoints in online mode.
In the ST editor, a possible breakpoint position is indicated by a gray filled circle:
You can restrict the breakpoint positions via the breakpoint.
of eachA set breakpoint is indicated by a red filled circle:
A disabled breakpoint is indicated by a gray circle with a red border:
When the program has stopped at a set breakpoint, this is indicated by a yellow arrow and yellow shaded line: