@rootnoun

A rootnoun is nounbase that looks like a verb root with no affix added, or with a /tuk-augment added.

Most rootnouns mean the doer —

/nI- "leader" from root nI नी "lead"

Most of those can be used only ifc after their object, after /su' "good", and after /dus "bad" —

**grAma- ग्रामॱ + /nI-
→ /grAmaNI- mf "village-leader", "maior"

Roots ending in a short vowel get the augment /tuk by hrasvasyapit... —

jit- जित् "defeater" from ji जि "be victorious"
indra-jit- इन्द्रॱजित् mf "Indra-vanquisher", "(s)he who defeated indra इन्द्र"

pustaka- पुस्तकॱ + bhR भृ + some /kRt affix → **pustakabhRt- पुस्तकभृत् mfn "book-carrier"

su-kRt- सुॱकृत् mf "gooddoer", "benefactor", "good maker", "skilled craftsman"

Although rootnouns appear to carry no affix, all of them are formed with a /kRt affix that loses all of its letters — maybe /kvip or /kvin —

spRz स्पृश् root "touch" + /kvin → **spRz- स्पृश् nounbase "toucher"

The grammar term for rootnoun is /dhAtu.

How do I know if the /dhAtu word in a sUtra means a true verb root or a rootnoun?

/pANini does not bother to tell, so he assumed that we would be able to figure out from the clues which is which. As for instance in sUtra eranekAc..., /dhAtu may mean both, but in sUtra lasya, only true verb roots are meant, which is sort of commonsensical.