For example, I want to set up two different set of criteria(for example spot and wavefront) to optimize. how can I switch between these two?

Method 1: Create two set of merit function combinations, then load the merit function

Method 2: Use GOTO or ENDX to swtich (change the value of GOTO)

You can find more interesting operands here, please enjoy:

The Optimize Tab (sequential ui mode) > Automatic Optimization Group > Merit Function Editor (automatic optimization group) > Optimization Operands by Category > Merit Function Control Operands 

Another example is BLNK, you can comment and this helps to locate the operands group easily.