I’m not sure about task types, but on an individual activity, if it is fixed to resource and fixed to schedule/time, PSO should not optimize / reschedule it.

Lily Ligocki

You could set the Base Value for these work types to 0. 
 

If you need this more flexible or only based on certain conditions, you could also create a small BPA that sets the “Exclude form Scheduling” flag on the work task.

Depends on what is expected from PSO regarding these manually scheduled tasks.  If PSO needs to be aware of them so it will schedule around them and not on top, then “exclude from scheduling” will likely not accomplish that.  And setting base value of 0 will allow PSO to be aware of them, but it will likely unallocate them.  So what I’m thinking is that they could be “exclude from scheduling” until manually dispatched, at which time they need to be included in scheduling but fixed.  I’m not sure how to accomplish this in FSM, but I hope this helps.