A service address is an address which can be used as an alternative to a residential address for the purpose of receiving post.
A service address is the official contact address that a company director, secretary, subscriber, Person with Significant control (PSC) or LLP member uses. This address is where you'll receive official mail from the government. It will also be listed on the public register.
Do note that the service address differs from the registered office address (both of which are required for registering a limited company). The latter is the official contact address of the company or LLP.