where count(A,B) is the number of times A and B co-occur in the entire document collection, and count(A) is the number of times A occurs with anything else. Intuitively, given a document collection, the relative frequency captures the proportion of time the word B appears in the same document as A.
Note: It is not very meaningful to consider a word (A) that only appears once in the entire document. Hence, you need to first rank all words according to their use frequencies (using WordCount) and then consider only top 100 words for the RF calculation of their pairs.