Wide-field VLBI is a term to describe observations using Very Long Baseline Interferometry (VLBI) techniques targeting several objects at one go. Historically, VLBI observations were limited to tiny fields of view (of order arcsec), because beyond a few arcsec the coherence of the data decreases and so the signal of a source would be degraded or lost.
However, this was not a fundamental problem, but a computational. Recent advances in computer technology have made it possible to (i) increase the spectral and temporal resolution sufficiently to circumvent the above effect, and (ii) to create multiple phase centres so that many objects (the coordinates of which are known) can be conveniently observed at once. See the paper "DiFX2: A more flexible, efficient, robust and powerful software correlator" (Deller et al., PASP, 123, 275) for details, and a paper describing a different approach for wide-field VLBI.
DiFX's Multi-phase centre capabilities are the primary advantage for a survey project such as bm360. From each observing run we obtain hundreds of data sets - one for each radio source which is known from previous VLA observations. These data sets can then be treated just like normal VLBI data sets, and calibration is carried out using ParselTongue scripts.