Battery voltage tells the SCC to provide power. As an example;
Battery = 26.5V
SCC = 28V max charge setting
Solar = 500W
If there are no loads then battery will charge up to 28V using 500W.
If there are 300W of load then battery will still charge to 28V using 200W.
If there is 700W of loads then battery will discharge as it is there to make up the 200W shortfall.
What SCC allows the battery to discharge if power is available? Makes no sense. You can only charge the battery if solar is greater than loads. Whenever battery voltage is lower than SCC setting the SCC will provide charge.
I'm not sure if this is a language barrier or what, or maybe you just have experience with a single system.
The vast majority of SCC have phases of charge - bulk (constant/max current), absorption (constant/max voltage) and float (arbitrary voltage where you want to the battery to be held at or near full and the charger will provide whatever power is necessary to maintain this voltage).
In your example, once your 28V max is reached, what happens? The battery can't continue to take the 500W forever. It can only take as much as it needs to maintain the 28V, thus current will taper. THIS is called
absorption. If this charger works in only this way and it keeps it pegged at the 28V regardless even if the battery can take only a tiny current, this isn't particularly healthy for batteries and goes against the charging method described by most LFP manufacturers.
Most SCC also have some absorption termination criteria where it either drops to float or stops charging. Sometimes it's based on time, a tail current or both.
To answer your one question:
A SCC that does not have an appropriate float voltage specified because that's what's telling the SCC to provide charge.