STA's primary function is to check a data signal timing to a clock signal timing, such as setup and hold constraints that require the data signal to remain stable around the active clock edge. In certain cases, we need to constrain the data change not to a clock event but another data signal event. These are called data-to-data checks. You can find them frequently in hard macros with asynchronous interfaces; but also in flip-flops with both asynchronous set and reset to enforce priority of one over the other.
Data-to-data checks can be expressed as user constraints (set_data_check) or as Liberty timing arcs
