Hello! Tilia is a genus (https://en.wikipedia.org/wiki/Tilia); the actual species can be e.g. Tilia cordata, Tilia platyphyllos, Tilia tomentosa, etc.

The spelling "<Genus> sp." is a common way to indicate a species of a given genus without providing the specific species, but since in OSM we do have the tag "genus" precisely for that case, it's better to use the tag instead of relying on spelling conventions of the binomial name. Cheers!