Currently, invitations and relay offers allow the sender to specify specific path types and (indirectly, in the case of invitations) signature schemes they're willing to support. However, some signature schemes might have large keys, or long computation times, or some other feature that makes them less suitable than other signature schemes for use with long or otherwise heavy paths. Therefore, it would be sensible for invitations to include a list of (public key, specific path type) pairs, rather than two separate lists (and for relay offers to do likewise with signature schemes and specific path types).