Note that x isa A and A <: B means that x isa B, eg
julia> map(T -> 5 isa T, (Int, Real, Number))
(true, true, true)
The manual tells you what an abstract type is:
Abstract types cannot be instantiated, and serve only as nodes in the type graph, thereby describing sets of related concrete types: those concrete types which are their descendants.
Again, I would recommend that you read the manual and allow some time to digest it. Figuring out these things by trial and error may be a somewhat confusing approach. YMMV.