Adding the same series/collection will give different results depending upon whether you use a Commentary or a Collection (see upper left). A Collection executes a basic search in the collection for the reference (see lower left). A Commentary executes a milestone search selecting the first occurrence in each resource of the collection (see right side - upper and lower). It therefore follows that only collection of resources with a Bible index may be used in a Commentary section while any text resource may be used in a Collection section.
Orthodox Bishop Hilarion Alfeyev: "To be a theologian means to have experience of a personal encounter with God through prayer and worship."