What I meant was different tariff structures for more passengers or larger vehicles, rather than 'extras'.
For example, a T3 that would apply if there were more than four passengers, say.
Basically, there's two ways of allowing higher fares for more passengers - extras, or a higher tariff across the board. (Of course, not impossible that there could be both on the same tariff card, but not aware of any examples.)
But that would mean the driver would change the rate depending on the amount of passengers, so to that degree a time-controlled meter wouldn't be possible, or at least the driver could easily abuse it if he could change the tariff with a couple of presses of a button.
Of course, the rationale behind time-controlled/calendar meters seems to be to stop drivers charging T2 during the daytime just by simply changing the rate manually, thus the meter can't be changed manually and instead automatically changes to T2 at the appropriate time.
So to that extent, if the driver can change from T1 to T-whatever to account for different passengers, then obviously that's open to abuse, and undermines the principle of time-controlled meters.
But thanks for the explanation of how the extras work
Although, to be fair, I've had my meter about 15 years now (and it was second hand even then) and still don't know how to use the extras button
Our tariff structure is very straightforward, just T1, T2 etc, and although there's a £2 extras charge for an 'abnormal load', I've never had the balls to charge it

, hence never actually used the extras button.
(Ironically, reason I got the meter for about £10 roughly 15 years ago was that I think Dundee was making time-controlled meters compulsory, so our Digitax man had a glut of older meters going cheap.)