I think I know now to what you referred.
There is actually a field ZUSCH called status key in table MCHA.
And if you press F1 for this field in SE16, then you get even a help that says you can further define own status keys for a better distinction.
This ZUSCH field is hidden.
I actually referred to the visible status field ZUSTD, which can just be a X or blank, where X means restricted.
The Data dictionary actually says the field ZUSCH is inactive. I found an OSS note 617285 - Status key is not displayed which confirms it as well and explains that this key is not used in standard R/3 system.