Leaders are the strings of characters (usually dashes or periods) which separate items in column-formatted lists. The WWP does not encode leaders of any sort.
If we were to encode leaders, we would use the rendition ladder keyword “fill” and the argument #leader. This information is encoded as part of the rend= attribute of the second element of the pair which the leader separates. This applies to any leader that marks the space, for instance, between a list entry (for instance, in a table of contents, cast list, or subscriber list) and a corresponding piece of information (a page number, role name, place, or price paid). We would specify only the presence of the leader, not the specific characters which make it up.