Post functions carry out any additional processing required after a transition is executed. You may find them useful in case you use manual Company defining (not auto-populating). The following are the standard Jira post functions the Company custom field works with.
Clear filed value
Use the standard Clear field value post function to set the Company field value to None on a transition.
Copy Field Value From Parent
Use the standard Copy Value From Other Field post function to copy Company field value to a sub-task from its parent on a transition.