Both from the questions currently tagged with this tag and from the meaning of the phrase summation method which I am familiar with (see Wikipedia) these two tags are very different. The tag summation is for finite sums. The phrases summation method or summability method are used for various approaches which assign some kind of sum even to series which are divergent in the usual sense.
We have two already existing tags where this meaning might fit. The tag excerpt for divergent-series currently says that this tag is for "questions on whether certain series diverge, and how to deal with divergent series using summation methods such as Ramanujan summation and others." This is the only tag I know of where tag-info explicitly mentions summation methods. I have also seen the tag regularization used for questions of this type. However, this does not agree with the tag-wiki and tag-excerpt.
The synonym was also mentioned in the Tagging chatroom.