I think "dynamic" means that you can replace the data or add data to it by use the D3DLOCK_DISCARD or D3DLOCK_NOOVERWRITE flags of lock() method.

I explain it in my mind are following:
If use D3DLOCK_DISCARD flag then D3D will assign a new pointer to the buffer and discard the data in it.If use D3DLOCK_NOOVERWRITE flag then D3D will hold the pointer then let you add data to it.

If use static vertex buffer then you can only fill it once.

Is it a right explain?

goddess just one...