Shirt pulling is not a mandatory caution and you often see minor shirt pulls during things like corner kicks. However, the large shirt pull where the shirt resembles a sail can often be an indicator of SPA; where it is not SPA, you can (and usually should) still caution these kinds of shirt pulling offences under the UB-Lack of Respect for the Game because it is a flagrant foul which can easily inflame and annoy the opponents.