path_adjust is used to either relax or tighten the timing constraints (even though defined by path_delay).
e.g to relax the constraints by 100ps
rc:>/ path_adjust -from [all_inputs] -to [all_outputs] -delay 100 -name pa_i2o
e.g to tighten the constraints by 100ps
rc:>/ path_adjust -from [all_inputs] -to $all_registers -delay 100 -name pa_i2r
path_adjust is thus use to guide RTL Compiler to concentrate and focus on register to register timing instead of optimizing inputs to output paths.
When such path_adjust exceptions are created there are located at /designs/my_design/timing/exceptions/path_adjust.