You can assign competencies to add companies within a trade.

Go to System Settings > Trades

Click on Assign Competency Requirements on the right of a trade.

Select a competency to assign to that trade:

The competency appears on the right:

Click OK.  

Select whether you want this to be applicable to individual projects or all projects: 

Any company that has been set up as having this trade, will now require this competency.  eg:  you'll see for the company below (Complete Carpentry), if you select the Edit Company button, because they have been assigned trade "Carpentry" they will have been given this competency requirement (National Certificate in Carpentry).

To set up a company with a trade, go to the System Settings > Companies (or Project Settings > Companies).  Find the company name and click Edit.

Add a trade:  

and Save.  

Now if you go to Manage Competency:

you'll see that because that company is part of trade "Carpentry", they have the following required competency: