Hi Stefan,
the PLC clock markers are directly represented in bit memory of the controller. Dealing with bit memory is problematic, since the size of the flag area of each controller is different. It therefore reduces the reusability.
The recommendation (also for TIA Portal) is to not use bit memory but rather to use a programmed block as clock generator instead of the clock markers. Clock markers are basically a legacy compatibility thing..
For SIMATIC AX, you can also implement a reusable Clock/Frequency block. You could have a look at the LGF_Frequency for some inspiration.