This function (termed "range" by Paul Nation) comes automatically with any word-list.
In any word-list you will see a column headed "Texts". This shows the number of texts each word occurred in (the maximum here being the total number of text-files used for the word-list).
The point of it…
The idea is to find out which words recur consistently in lots of texts of a given genre. For example, the word consolidate was found to occur in many of a set of business Annual Reports. It did not occur very often in each of them, but did occur much more consistently in the business reports than in a mixed set of texts.
Naturally, words like the are consistent across nearly all texts in English. (While working on a set of word lists to compare with business reports, I found one text without the. I also discovered that one of my texts was in Italian: but this wasn't the one without the! The culprit was an election results list, which contained lots of instances of Cons., Lab. and place names, but no instances of the.)
To analyse common grammar words like the, a consistency list may be very useful. Even so, you're likely to find some common lexical items recur surprisingly consistently.
To eliminate the commonly consistent words and find only those which seem to characterise your genre or sub-genre, you need to find out which are significantly consistent. Save your word list, then use it for comparison with others in WordList, or using KeyWords. This way you can determine which are the significantly consistent words in your genre or sub-genre.