A splice is the point where two chord members are joined together by a metal connector plate (truss plate). It can occur at a panel point or between panel points depending on the specific design for that truss and the preferences of the truss technician and/or truss manufacturer. Splices allow multiple shorter pieces to be joined together to form a longer chord member and is a more efficient use of lumber.
Chord size can be changed (2x4 to 2x6 or greater) at a splice point if so desired either to meet engineering requirements or architectural effects. An example of this might be an attic truss where the bottom chord is increased in the living space where additional loads are applied and then reduced near the bearings to reduce expense. Another example is an oversized tail in an exposed overhang that then reduces to a more efficient chord nearer the peak.
Splice length is determined by the truss technician based on lumber inventory, span, and cutting preferences. Certain manufacturers determine splicing based on their equipment capabilities and will either splice from the center out or from the outside in, depending on what is most efficient. There’s no right or wrong way to splice, only manufacturer-specific best practices and preferences.