IcU is the maximum breaking capacity of the device yours is 25ka, this is it's limit and if it disconnects with a fault above 25ka theoretically it should be replaced. IcS is the breaking capacity to which the device can operate and be reused, it shouldn't cause damage and as you say this is normally a percentage and yours states IcS is 50% of IcU so yes 12.5ka. Again if the device disconnects above 12.5ka in theory it should be replaced.

If you know the supply PSCC at that DB you can work out if it ever could have hit a damaging level of fault current. I suspect if it has there would be some signs of metal vapour having been expelled from the device's arc-chute inside the panel.

There might be some HRC fuses as well at the supply point and sometimes they will provide protection against damage if you are looking at tens of kA fault levels (as the fuse might be current-limiting at that sort of level) but that would need a bit of checking.

In terms of time, let-through, 5s Zs, etc, you would need to get the data sheet and also check what the adjustable settings are. From your picture I don't see any rating, that might be hidden as there is some of the adjustable section visible. This might be only a thermal adjustment (so your long-term limit can be set from 100% down to 80% or similar) but some also allow you to set the short term trip points as well to better coordinate with downstream breakers.

That only has a thermal adjust of 63 or 80A.

Just spotted the TM 80D code, are they one of brands Schneider have borged? It looks a bit like an old Merlin-Gerin NS100N come to think of it. If so you might get data more easily and/or ask Schneider's technical help folks.

We'll need to see the settings on the tripping unit, it's partly obscured in the picture.

The part number TM80D on it is suspiciously similar to Merlin Gerin tripping units. Plus it generally looks a lot like an older Merlin Gerin unit so I'm wondering if it was made by them and badged by federal.
If you are then the datasheet will be easy to come by.

Many thanks for all of the replies. You lot are great.
I'll take a better photo next time.

My main confusion was lack of any kind of rating but it sounds as though with the cover off more should become evident.
There are indeed upstream supplier fuses.

The PSCC at the downstream DB is 2.7kA (phase to phase) so I don't have concerns about exceeding it's capabilities on that front.

It's more having some means to check the 5s disconnection time and CCC of cable.
Anyway, I'll get the cover off and report back.

word of caution, the federal electric breakers are old bits of kit.
I have known them sometimes to refuse to turn back on if they are tripped or even just switched off.