In both Spectral Line and Single Continuum cases, it is possible to enter sources with different velocities. For Single Continuum, this is usually irrelevant as the setup is usually defined in the sky frame and the source velocities are ignored. For Spectral Line though, the velocities are crucial as these are used to convert the rest frequencies to their observed equivalents. In general, this means that each source requires its own tuning and its own set of calibration observations. This greatly increases the complexity and time required to complete a project and thus each Science Goal is restricted to five separate tunings.
The OT automatically calculates how many tunings are required by calculating a solution separately for each velocity and then seeing which of the other sources could be observed using this tuning. The result is a minimum list of tunings required to observe all the spectral lines. The number of tunings is reported in the Time Estimate dialogue (Section 5.3.5.3).