Great question, maybe worthy of its own post.
Queue a Background Logic Block will always queue up the specified logic block to run in a separate background process. It provides the additional “Queue Immediately” checkbox that you need to specify behavior specific to queueing.
For Call A Logic Block, the difference arises based on where the calling logic block is running:
- If the calling logic block is running synchronously (e.g. as part of a Field Value Changed action), the called logic block will be queued to run in the background.
- If the calling logic block is running asynchronously (e.g. as a Job or part of an automatic Workflow transition) then the called logic block will run in-line, since it is already in the Background.