There are multiple options to consider Cable Sagging in RM Bridge, the user choice should be related to the model needs according to the problem relevance, namely its size and complexity. 

Initially, the Elements Type Cable is used to have Normal tension forces only and give linear results. Tough, cable sag is force-dependent and therefore non-linear! The loads are the normal pre-stress loads and other elements loads, together with the transversal loads, like self-weight and wind. So, for cable elements with very small bending stiffness, the deflection will depend a lot on the level of stress, and that is very variable in the construction stages.

Therefore, additionally, the Action CabSag should be used to create the non-linear behavior simplification that is modeled with an equivalent E-modulus taking into account the sag. This action is a correction of the elements Young's Modulus E by a specified factor creating an Ernst Modulus E*. The cable sagging should start from a stress-free element geometry (L0) and consider the deformed geometry under load.: L0=EA*L/(EA+N)=constant 

The Non-linear stay cables Recalculation option can be selected if an analysis with full nonlinear cable sagging is needed. With this option, all cable elements are calculated using the large deflection theory (IIIrd Order). This is the exact non-linear calculation, it requires the elements Cable to be subdivided, either internally or at their development, to consider the deformations. A subdivision into 8 cable elements is generally enough.

More information can be found at the links below and the product examples and documentation.

communities.bentley.com/.../rm-bridge-cable-subdivision

communities.bentley.com/.../cable-forces-fcab-fx0-or-lx0-and-cable-sagging