Other than the difference on total applied load, what is the main differece between the two loads? When is more approppriate to apply one or the other?
HS25 is a 45T truck with the same axle configuration as HS-20. It came about 20 or so years ago to address the presence of heavier trucks and tandem vehicles. The decision to use HS-25 or HS-20 is up to the bridge owner. However, if you're designing a new bridge the requirement may very well be HL-93.
HL-93 is the vehicle loading for LRFD. The HS-20 vehicle loading is for LFD. Essentially, they are the same loading except HL-93 requires the lane load to be applied simultaneously with the truck load. A simple internet search can find the size of the truck and weight of the axles, or in the LRFD Specification.
HL-93 is an HS-20 truck (4T axle and 2-16T axles, usually spaced at 14') with a 640 plf lane load. This is a bridge loading, which is also applicable to box culverts. In NY there are certain instances where we neglect the lane loading on box culverts and manholes. I'm not sure what others do for a buried pipe with regard to lane load but it seems like an overkill.
Note that the load factors for LFD and LRFD may be different. They are calibrated for probability of failure for LRFD, which is why FHWA pushed all DOTs to adopt everything LRFD, even load rating on LFD bridges.
