ELEMENTAL is relevant to DO CONCURRENT only indirectly. The ELEMENTAL attribute matters there only because it implies the PURE attribute by default, and PURE is required for procedures referenced in DO CONCURRENT. (Which is not a parallel construct, but that's another matter.)

ELEMENTAL in array expressions (incl. FORALL) should not be understood as being a way for one procedure invocation to receive and return entire arrays as arguments and results. That would require buffering during the evaluation of an array expression. Instead, ELEMENTAL should be viewed (and implemented) as a means of allowing a function to be called as part of the implementation of unbuffered array expression execution.

ELEMENTAL has its roots in compilation for true vector machines. It once caused a function to have multiple versions generated: a normal one with scalar arguments, and a vector one with vector register arguments. This would allow a user-written ELEMENTAL function to be called in a vectorized DO loop, just like an intrinsic vectorizable function like SIN. A compiler for today's SIMD "vector" ISAs could implement ELEMENTAL in a similar fashion.