On aircraft Hobbs are oil pressure activated, hence give 'total running time'
The Tach is RPM related hence at idle is derated but true when at operational RPM.
The Hobbs is used for 'flight time' (what the pilot logs in his book.
The tack hours are used to determine engine Overhaul hours.
Now when you rent by the hour they charge you by the Hobbs time but declare the engine 'Tach' in the books.
That way the have a win-win situation.
I knew of a large project where they would idle the cranes all day and charge for the full 10 hrs (Hobbs Time) but were insured by the working hour (Tack time) as that was when there was real risks.
The auditors told me that a typical crane with , say, 8000 Hobbs (revenue hrs) had only worked maybe actual 2000 hrs.
That was at the Expo 67 construction sites.
We will have finally paid off the construction debt this year, but the stadium now needs a new roof! (The third one)