Question:
What is Spot Pay for? What is "Spotting"?

Answer:
Spot pay is pay for spotting (yes, I am a smartazz)!
Spotting is moving a trailer, usually a short distance. Say, for example, your company has a couple of customers in the same city and the company leaves empty trailers at each customer (to be loaded at the customer's convenience). Customer A may run short of trailers and your dispatcher may send you to Customer B to pick up an empty trailer and "spot" it at Customer A.
Hope that helps.

Answer:
"Spotting" can also take place within the Same customer's lot.(if the customer does Not have a "shag" drvr or trailer jockey.)
The customer may have several of you company's trailers in the docks/doors being loaded/unloaded....since the customer does not have a shag drvr or trailer jockey of their own, they call your company & ask them to send one of their drvrs to pull the empty trailers out of the docks & put a loaded trailer that has been dropped in the yard in a dock.(or vice-versa...they call your company to have a drvr pull a loaded trailer out of the dock & put an empty trlr in the dock.)