I have a very generic question regarding explicit element formulation in ABAQUS (VUEL). We need to define RHS (element internal force vector F_int), amass (diagonal or consistent mass matrix) and dtimestable (defining stable time increment) for an user element to work. I wonder why we dont have to define the external force of the element which is F_ext? To be more clear, the second law is Ma = F_ext -F_int, or a= M-1*(F_ext-F_int). Here F_ext contains the body force contributions and surface tractions.
My question is, why is it that in abaqus, we dont have to define F_ext and defining F_int is just sufficient? How does abaqus integrate then the external surface traction on the elements?