‘Now these are the generations (family histories, records) of Aaron and Moses in the day that Yahweh spoke with Moses in mount Sinai.'
This is probably the remnants of a colophon closing off this military record. A colophon was included at the end of an ancient record in this way just as we would put a title and author's name on a book cover. It gave a quick reference to the contents of the record. As we saw earlier, in Numbers 1 & Numbers 2 Moses and Aaron were seen in this section as continually acting together apart from when Moses was receiving Yahweh's direct commands (see on Numbers 1:2-3). This verse is not really suitable as a heading for what follows, for Moses does not feature there, but it does make a very suitable ending.