A source as defined by the OT is a collection of parameters comprising the source coordinates, its proper motion, radial velocity, and expected source properties (line/continuum peak flux density, line width and polarization percentage). It may have a target type 'Individual Pointing(s)' or '1 Rectangular Field', and contain one or several pointings.
The pointings defined within a source (whether as potentially multiple, overlapping pointings or a rectangular mosaic) all share the properties of their parent source; the only way in which one pointing differs from the next is the RA and Dec. In particular, they all share the source velocity information and will therefore be observed with the same frequency tuning. All the pointings making up a source definition will be combined into a single image by the ALMA Pipeline and must overlap. Different sources on the other hand are imaged separately (even if their pointings happen to overlap).