Post functions are run after the transition has been executed. When you add a new post function, you might notice that the transition already has a number of post functions pre-added; this is shown in the screenshot that follows:
These post functions are system post functions that carry out important internal functions, such as keeping the search index up to date. The order of these post functions is important.
Always add your own post functions to the top of the list.
For example, any changes to issue field values, such as the one we just added, should always happen before the Re-index post function, so by the time the transition is completed, all the field indexes are up to date, and ready to be searched: