Is there some detailed documentation available for "simple queue" ?
The WiKi and the help site only give one example of "say you have this then you set it like this" but this does not explain most of the parameters...
If possible I would like to setup a queue for a guest WiFi, where each user (on a bridge) gets an absolute maximum ul/dl rate, and the users all together consume a maximum ul/dl rate.

I'd be curious at the answer myself, I understand queue trees better. If you want to throttle lan clients ONLY, then I know that part works as advertised: put the guest LAN subnet as target, and set the max values. That throttle per-client (e.g. per IP) bandwidth.

But when you starting dealing with needing parents (e.g. the "overall" part of your request) that were I'm not sure what's the best approach in simple queue.

Indeed, that is what I wonder about. There is a "total" field on a separate tab, but it does not mention upload/download as the other fields do. So what is it for?
I'm also still not sure if the "target upload/download" values are really separate for each client, and if so what will happen when several clients each request data at a rate below max-limit, but together they are above the internet speed. Will the bandwidth be divided equally amongst the clients? Or is that only for the limit-at rate?
There should be at least a description of what the fields do, and how the queue is structured.

Thank you for the feedback, we will eventually add more examples, feel free to mention other topics here, that should be expanded on.

Well, before adding more examples, please start off with a brief explanation of what the command does (does it set up a separate queue for each target?) and a description of all the parameters as related to that.
E.g. what parameters are for individual targets, and where does the "total" fit in. And what are the packet marks and the priority used for.

A list of parameters with a description sure help a lot, the docs just skip that part in Queue. In particular, what exactly is meant by "total"?

Is "default-small" really the best choice for queue type in @pe1chl example?

In general, some "Selecting the Right Queue Type..." section be helpful, from y'all POV. e.g. For guest Wi-Fi in some QuickSet modes you can set a bandwidth limit, this doesn't actually use any queue, but 802.11 limits, is that better than simple queue?

I would second the Author's concern about more documentations on "Simple Queues"
Say for example, what is the max allowable rate limit you can set?
In my case I tried setting at 10G/10G but it does not recognize it, but when I put 10000M/10000M it would recognize it as 1410065408/1410065408.

The value somewhat overflows in the internal computing of ROS v7(10^10)

Regardless the limit of 0xFFFFFFFF (4294967295 (~4GiB), simply G is not a supported letter. Just M and k.
(yes, I know, must be writed)
RouterOS have everywhere 32 bit counters because the CPU used at the start are 32 bit.
Now are present arm64 and other 64 bit cpu, but is still present limits till 9223372036854775807 (~8EiB)

So 10GiB is > of the max ~4GiB (for write that on queue, write 4294967295 without M or k).
Use ARM64 device, and you solve the 4GiB limit problem....

If you use terminal is explained

example on v6 code

[] /queue simple> set 1 max-limit=4294967295/4294967296
value of download-max-limit out of range (0..4294967295)

[] /queue simple> set 1 max-limit=?

MaxLimit ::= UploadMaxLimit/DownloadMaxLimit
  UploadMaxLimit,DownloadMaxLimit ::= 0..4294967295    (integer number)

On RouterOS the documentation and the software is wrong because must not accept numbers > 9223372036854775807 (0x7FFFFFFFFFFFFFFF)
(Because after 0x7FFFFFFFFFFFFFFF there are negative numbers from -1 to -9223372036854775808)

example on v7 with ARM64 CPU code

[] /queue simple> set 1 max-limit=<F1>

MaxLimit ::= UploadMaxLimit/DownloadMaxLimit
  UploadMaxLimit,DownloadMaxLimit ::= 0..18446744073709551615    (integer number)

#### REX note:
#### WRONG !!!! is 0..9223372036854775807 #### 

Is a bug on RouterOS that do not check that, but also 8EiB can be sufficent.
Some prediction on 2022 suppose that all the world use 273EiB fixed and 77EiB mobile traffic...