Map an explicit (legal) zone, as opposed to some other area where maxspeed:type would apply? As I said, this might not apply in your country, but it might in others.
Can you give us an example in the map?
What is the difference between zone:maxspeed:DE:30 and maxspeed:type=DE:zone30 ?
It both describes the same maxspeed zone. Or do you use aditional attributes to identify the zone?
In that example there clearly isn’t a difference, because you’ve moved the “this is a zone” information to a value of the “maxspeed:type” tag.
Your idea is that you can already filter zones by key? That was my first idea of zone:maxspeed too, but do you know that zone:maxspeed is also intended to be used with values like xx:urban and xx:rural?
This means that all roads without a speed limit sign are in one zone.
And there is no homogeneous usage. I always end up having to read all 3 tags to gather the information I could have in one.
No - the UK has a concept of “zones”, and also a concept of “not in a zone”, and also rules for what the limit should be if there is no explicit signage (rare now). Legal maxspeed zones in the UK are fairly limited and most roads are not in them.
Is this documented in the wiki?
No, here. Note the reference to “20 mph limits and zones” on that page. Whether it makes sense to make it a hard rule to document which category a particular speed limit falls into is another question, since as mentioned above it’s not always straightforward to check.
I dont mean the traffic rules, I mean when to use maxspeed:type and when to use zone:maxspeed in UK.
No, it’d need a proper discussion first. As I said above I’d be worried that it might be too onerous for people and might actually drive down contribution of useful data. For the avoidance of doubt, there isn’t a problem in the UK - usage is pretty sensible.
Only 91 uses and more than half with non-compliant values and you think this tag is very important in UK?
This was my (not vary diplomatic) try to start a discussion about zone:maxspeed vs. maxspeed:type. Be sure, I don’t intend to simply retag the values.
Actually only around 30 are legally plausible 
However, it is a UK legal concept and I suspect that there will be a better attempt to map these in the future. What we wouldn’t want to do would be to think that zone:maxspeed and maxspeed:type are analogous in the UK - they clearly aren’t.