Field help (STDLABWST) 

A labor work center can be associated with the main work center which is of machine type.
The labor work center times are calculated via two coefficients:

When creating a routing operation, the site attached to the work center must be the same as the site dedicated to the routing code, provided that the latter site is mentioned.

If the main work center is of sub-contracting type, no secondary work center can be associated.