Rainwater use with cistern: rainwater retention

Hello everyone,

we are currently looking for the appropriate cistern which we will use only for rainwater and subsequent garden use.

Since we have to retain rainwater and ensure that it drains off slowly, I have become aware of retention tanks. These have a useful volume and a retention volume. If we now need a cistern of 3000 liters due to the sealed surface area, is that only the useful volume? Or is it the useful volume + retention volume?

And then a technical question:

In the picture, the useful volume and retention volume can be seen and are separated at the level of the float. What happens if it rains so heavily that the retention volume is also exhausted and the water flows into the overflow from above? When the rain then decreases, does the water level drop to the maximum useful volume, i.e. is the float able to drain? Or is the retention volume also maintained until it is basically withdrawn?

Do you know what I mean?

The water always drains down to the level of the usable volume. Otherwise, it wouldn't make sense. I just don't understand why the thing needs a float.

Only 3000 liters? We already need that much for a carport here.

Do you know what I mean?

No, no idea. For some unknown reason, a shut-off valve was installed before the throttled outlet on my side. But it seems to be somehow clogged or "adjusted."

Judging by your explanation, you understood what I mean.

That's exactly how I thought it too.
Normally, you have the usable volume; if it rains heavily, it rises to the retention volume and overflows at the top.
Then it slowly decreases.

What is the purpose of the float: I wonder that too. Although, is that a float or just a throttle?
Because the delayed discharge is given anyway if you set the throttle accordingly.

Yes, according to my information, we need 1000 liters per 100 sqm of developed area.

Although I am still clarifying whether it is 1000 liters retention volume or usable volume per 100 sqm.
The fact is, we have rainwater and wastewater separated in the sewer and must ensure delayed discharge.
This is ensured by the system. Therefore, I also think that the 1000 liters refers to the retention volume, because the usable volume probably doesn’t matter to the authority and only concerns me.

We have a large garden and will probably go for 5000 liters usable volume + 2000 liters retention volume.