Vocabulary, not instructions. Electrical, gas, structural and roof work can injure you or burn the house down, and much of it legally requires a permit and a licensed trade. Knowing the words is for describing the problem accurately — to yourself, to the store, or to the person you hire.
What you'd probably say, and what it's called on the shelf. The right word turns “the thingy under the sink” into a two-dollar part you can walk in and buy.
Trades divide sharply, and the boundaries are enforced by licensing. Calling the right one first avoids paying someone to tell you it isn't theirs.
The working vocabulary, by system. The last group — hiring and paperwork — is where most money gets lost on home projects.
The numbers that come up constantly, including the ones that aren't what they say on the label.