You can add dependent fields to an action based on the action category, so for example if the action is a legislative action, you could then see a "Related legislation" or "legislative source" field.
To do this click on the admin gearwheel in the top right then select layouts. On the layouts page select Action:
To add a dependent field, first click on the field that the other field is dependent on, in this case that being the Action Category:
Then on the next page click on the Dependent button next to the answer you want to add a dependent field to, and use the search pop-up to select and add the field.
Now depending on what category the action is put into different fields will show up on assignment.