From the BLS website (this would be the US, but applies to other countries as well):

> Some businesses do not produce goods for sale, but instead provide services. It is more difficult to count the number of units of output in such businesses. > > legal services > haircuts, beauty salons > mechanics and repairs > medical and dental services > > Often, businesses produce more than one good or sell more than one service. It is difficult to add up, for example, 300 chairs, 60 tables and 100 beds to express the total output of a furniture business. > > Therefore, for most goods and services, we measure output by the dollar amount sold. Expressing output in dollars makes it possible to measure goods and services that are difficult to count, and to add up different types of goods and services.

Public sector jobs (such as teachers and bus drivers) would normally be excluded.