Section (10-14 to 10-19) of the following manual mainly discuss Master Control Relay in STL.

http://support.automation.siemens.com/WW/view/en/18653496

Errors in programming or lack of full consideration while using MCR may cause CPU to stop . read these notes before using these set of instruction in your program. Also note this is not a good alternative for hardware emergency stop or safety system.

Danger: PLC in STOP or undefined runtime characteristics!The compiler also uses write access to local data behind the temporary variables defined in VAR_TEMP for calculating addresses. This means the following command sequences will set the PLC to STOP or lead to undefined runtime characteristics:Formal parameter access· Access to components of complex FC parameters of the type STRUCT, UDT, ARRAY, STRING· Access to components of complex FB parameters of the type STRUCT, UDT, ARRAY, STRING from the IN_OUT area in a block with multiple instance capability (version 2 block).· Access to parameters of a function block with multiple instance capability (version 2 block) if its address is greater than 8180.0.· Access in a function block with multiple instance capability (version 2 block) to a parameter of the type BLOCK_DB opens DB0. Any subsequent data access sets the CPU to STOP. T 0, C 0, FC0, or FB0 are also always used for TIMER, COUNTER, BLOCK_FC, and BLOCK_FB.

Check the attachment for more information.