The sphere of influence of a service is how far people will travel to make use of that service. A primary school, which is a low order service, will have a smaller sphere of influence than a secondary school which is a middle order service. People are willing to travel a long distance to get to a high order service (e.g. Harrods shop in London) and it therefore has a very large sphere of influence.

Settlements as a whole can also be said to have a sphere of influence. People will usually only travel a short distance to a hamlet (e.g. Levenwick in Shetland) but will travel further to a town (Lerwick in Shetland).