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    $\begingroup$ In addition to the answer of @Mr.Wizard, I discussed exactly this issue in this answer (last subsection), where it is put in a wider context of UpValues in general. $\endgroup$ Commented Jan 25, 2013 at 10:11
  • $\begingroup$ Tks @LeonidShifrin, It was very helpfull. $\endgroup$ Commented Jan 26, 2013 at 0:52
  • $\begingroup$ Murta, I see that you did not Accept my answer. Is there something I can do to make it more satisfactory? $\endgroup$ Commented Feb 27, 2013 at 8:31
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    $\begingroup$ @Mr.Wizard Re: UpSet vs TagSet - the only practical difference I am aware of is that TagSet is more precise, and will only add an UpValue to a symbol you specify, while UpSet will add UpValues for all symbols at the first level in the l.h.s. of the rule. Re: email change - the funny this is, I didn't change it! Apparently, SE changed the algorithm they use to compute at least some gravatars. I've noticed it and some other users seem to have been affected by it as well (although not all, it seems). $\endgroup$ Commented Sep 1, 2016 at 12:50
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    $\begingroup$ @Mr.Wizard There certainly is a difference.When you execute e.g. ClearAll[a, b, c, d]; f[a, b, c] ^= d;, all of the a,b,c get UpValues, whereas when you execute ClearAll[a, b, c, d]; a /: f[a, b, c] = d;, only a does. With more complicated patterns involving blanks, it may matter. Generally, you want to be as precise as possible, to not produce extra global rules besides those you need, because they may fire in unforeseen situations. Actually, we pay for locality of overloading based on UpValues with making it harder to see why a given rule fired. No reason to make it harder still. $\endgroup$ Commented Sep 1, 2016 at 22:52