In fact the tag wikis of summation and sequences-and-series are not properly scoped, and even they are not named properly.
Sequence is an ordered list of objects (the objects can be numbers, functions, etc.), Series initially it refers the sum of the terms of a sequence, but in modern mathematics it even extended to refer the summation-type kernel function. The complicated type of series can contain several summation signs (e.g. http://en.wikipedia.org/wiki/Kamp%C3%A9_de_F%C3%A9riet_function). Summation only refer the operation of sum, the summation tag has an inappropriate tag name.
The questions about sequences are not necessarily interested about sum of the terms, while the questions about series are often interested about e.g. the existance of close-form, the convergence etc. rather than about e.g. the origin of which sequences, so the sequences-and-series tag also has an inappropriate tag name.
Even the tag wikis of the sequences-and-series tag and the summation tag are very awful.
The tag wiki of the sequences-and-series tag:
Recurrence relations, convergence tests, identifying sequences
Does't there already have the recurrence-relations tag specify for questions about recurrence relations?
The tag wiki of the summation tag:
Questions about evaluating summations, especially finite summations. For infinite series, please consider the (sequences-and-series) tag instead.
The first sentence and the second sentence are contradicted, since the first sentence said that questions about infinite series can use the summation tag, but the sentence sentence said that can't. Therefore no wonder why quite many people use both the summation tag and the sequences-and-series tag.
So the summation tag and the sequences-and-series tag should be deleted. The replacement should be to create the sequences tag specifically for the questions about sequences, the finite-series tag specifically for the questions about finite series, and the infinite-series tag specifically for the questions about infinite series.