In FLAC, there are two types of loading available to be applied on SELs - pressure and velocity loading. What type of loading should be preferred and based on what conditions should the loading type be chosen?
I am assuming this question is for FLAC3D and not FLAC since it is tagged as such.
One loading type is not preferred over the other. It depends on your application. For example, you can apply uniform distributed loads (force per unit length for 1D elements, and pressure for 2D elements) to structural elements using the structure <‘element-type’> apply command. Where element-type can be a beam, cable, shell etc…
Additionally, you can apply force or moment conditions to individual nodes using the structure node apply command (documentation here). Velocity or displacement conditions can be initialized with the structure node initialize command (documentation here).