Problem
You need to add new trades like plumbing to your Pricebook Pro.
Solution
Add a catalog
Go to the navigation bar and click Pricebook.
In the side menu, click Catalogs.
Select ServiceTitan Pricebook.

Under the Services/Equipment section, choose Commercial, Construction, or Residential.

To add all items for the trade, click the checkmark
next to the catalog. 
To select specific items, click into the catalog and check the ones you want.
Click Add to Pricebook to add the selected items.
For more, see Pricebook Pro Onboarding Part 3: Add provider catalog items to your pricebook with Pricebook Connect.