Unambiguous. That is how all instructions should be.
To me, heading is heading and track is track. End of story.
The FMS departure procedures, when flown in LNAV, contain both track and heading legs, as in the published ones. In the first, track is maintained. In the latter, heading is maintained regardless of wind.
Doing one way or the other may seem quite irrelevant most of the times, but in a strong crosswind day with paralel runways or a nearby airport or some noise sensitive area it is relevant to do it in the correct manner.