This is your database of parts. Your estimates will include physical things like wire or a fuse board but they'll also include time for people and equipment. Some parts are sold by unit (a number of X) and some are sold by the metre or another measure. You can configure all of these settings here in the parts list.
When you're building your estimate on site using the mobile app, it's made easier by categorising (or tagging) the things in the parts list. You can create tags to suit you so you might have a tag for 'labour' or 'consumables'. You could be more specific and have tags for 'fuse boards' or 'types of wire'. Set it up to suit you.
Once you've visited your customer and created their estimate a PDF is emailed directly to the customer for their consideration and, hopefully, acceptance.