Perturbatively, fundamental strings are more fundamental than branes or any other objects. In that old-fashioned description, D-branes are "solitons"… - Luboš Motl

" "

Perturbatively, fundamental strings are more fundamental than branes or any other objects. In that old-fashioned description, D-branes are "solitons" — configurations of classical fields that arise from the closed strings. They are analogous to magnetic monopoles — which may also be written as classical configurations of the "more fundamental fields" in field theory. In a similar way, D-branes' masses diverge for 𝑔→0. Non-perturbatively, D-branes and other branes are equally fundamental as strings. In fact, when 𝑔 is sent to infinity, some D-branes may become the lightest objects — usually strings of a dual (S-dual) theory. When we include very strongly coupled regimes (high values of the string coupling constant 𝑔), there is a brane democracy.