In Eyrus a contract is what allows a company to show up on a project. Tradespeople are assigned to a company and then a company can be assigned to one or more projects.
If Bruce Springsteen works at Acme Concrete and Acme Concrete is working on Project Artemis then Acme Concrete would need to have a contract for Project Artemis.
Eyrus' system support two level of hierarchy; the top level we call the prime contract and then there can be sub of subs underneath that prime contract. With the example above if Acme Concrete subbed out the rebar work to Initech Rebar then Acme Concrete would be the prime with Initech Rebar underneath.
To learn more about contract see the following articles: