In the wild, it becomes very quickly obvious that there are no inviolable rules in heraldry. There are certainly conventions and common sense which you ignore at your peril, but do not believe any one who suggests that there are any rules that have generally held good throughout the time when heraldry was a matter of practical daily use.
One oft-stated “rule” that the heralds themselves showed to be largely ignorable is that only one person at a time is entitled to use a given coat of arms, and that everyone else must differentiate his coat of arms in some way from the original. Even in the Heralds’ Visitations themselves, it is common to find Mr Armiger of Someplace given the same arms as Mr Armiger of Firstplace.
Where the same arms are used it is generally by two branches of the same family, but coats of arms bearing the same description might also be found used by members of different families. The fact that the great dispute between Lords Scrope & Grosvenor is so often quoted shows not that the rule was generally observed, but that such a rule was generally let slide. For any case such as Scrope and Grosvenor (if, indeed there are any other such cases) there must be hundreds of instances where people shared the same arms either not knowing, or not caring.
On the first page selected at random from about 1100 pages of Papworth’s Ordinary, I counted eleven coats of arms which were each used by multiple families. Unlike the Visitations, Papworth’s Ordinary is not a snapshot, so maybe not all of those had multiple owners at the same time, but some of them would have done. In other cases, it may have been that the coat of arms passed complete and undifferenced from one family to another, usually but not necessarily by inheritance. It is clear that coats of arms were sometimes seen as belonging to a place. This is obvious enough when we are talking about royal coats of arms, but it also applied to units as small as a manor. When, for example, a Chichester obtained the manor of Raleigh he adopted arms that had been used by the Raleigh family. Subsequent Raleighs used completely different arms.
In spite of my comments above, members of the same family often took the trouble to change their arms slightly “for difference.” Do not believe, however, that the carefully regulated marks of cadency supposedly used in Scotland were ever applied in the West Country. (Common sense will soon tell you that they have never been generally applied in Scotland either, as three or four generations would soon cause a complete jumble of unreadable coats of arms.) So, labels, borders, crescents, mullets, and annulets appear frequently enough added to a basic coat of arms, but without any discernible meaning as to the place of the holder in his family. Changes of colour and the edging of main charges are also fairly common.