NOTE: The drive letter is simply a recommendation. It can be any drive you choose if the suggested drive letter is currently in use.
Home (M:) is personal space for each user.
Office (O:) is shared space setup for departmental groups.
Research (R:) is shared space setup for various research groups
Think of DFS (T:) as a drive containing all the other drives. Mapping it should eliminate the need to map any of the other network drives.
Web (W:) is shared space for various groups dedicated for web sites.