The "principal place of business" is the main place where the business's business is/will be carried on (loosely speaking, "the headquarters" of the business).

The principal place of business for numerous small businesses is simply the home address of a individual / sole trader.

Even if the business sends out workers, to do jobs / a job on customers' sites, the principal place of business would still be likely to be the office/home address from where the jobs are co-ordinated and/or from where the "paper work" is done.

The "address for serving documents" is the address where a representative of the business can physically be handed a document. As such the address for serving documents is most commonly the same as the principal place of business and can not be a P.O.Box address.