I am trying to understand why the $D$-brane structure of a string theory matters for string phenomenology.
From what I understood from Wikipedia, the $D$-brane structure determines the gauge symmetries of the theory, the intuition being as follows: the position of the D-branes determines the minimum length of the open strings, and thus the possible vibrational modes. Since the vibrational modes correspond to the particles (including the gauge particles), we can deduce the gauge symmetries of the system from the particles that we can obtain in the $D$-brane configuration of the system.
Aren't $D$-branes dynamical objects? Then shouldn't the distance between them vary? (This argument seems to assume that it does not vary).