I found the following statement here (https://forum.simutrans.com/index.php/topic,1959.msg20214.html);
QuoteFixed maintenance costs
In Simutrans-Extended, vehicles can be set to have a fixed monthly cost as well as a per kilometre cost. This is achieved by using the "fixed_maintenance=" value in the vehicle's .dat file by the pakset author. The function is dependant on the pakset - using paksets designed for Simutrans-Standard will give the same behaviour in relation to running costs as Simutrans-Standard. From version 9.3 onwards, this cost will be reduced by 95% (to 5% of the original cost) when a vehicle is stored in a depot to represent the fact that most of this cost is the staff cost of running the vehicle, but some small element of maintenance is necessary even on a vehicle that is not used to keep it in a serviceable condition.
But there seems to be no such fact. (´・ω・｀)
(https://i.imgur.com/Nysuj72.png)
The fixed maintenance of this convoy is 170.20 ￠.
(https://i.imgur.com/jmppmTk.png)
This convoy has been in the depot for seven months.
I left this convoy from the depot just before the end of the previous month and 170.20 ￠ were collected at the end of the last month.
It seems that fixed cost has not been reduced while being placed in the depot. :-|
Thank you for your report. I suspect that that feature was accidentally overwritten without anyone (including me) noticing a long time ago - or perhaps even deliberately changed without the documentation being updated.
The question is now: since there is a discussion of modelling the staff costs as a separate figure to this fixed maintenance costs, would it be better to change the code or change the documentation for this? I suspect the latter.
For the present, I have added a strikethrough to this text in the documentation.