What are human-receiving stations?

The human intellect is capable of receiving communications from the first spiritual zone only. 

All spiritual communications from the higher spiritual zones must be received by the spirit-life direct—and be transmitted by it to the intellect.

Spirit-lives that are highly developed may receive direct spiritual communications coming from the second and third spiritual zones, but from beyond the third zone, they cannot. 

They must depend for further information upon relayed messages, that is, upon messages that have been received by at least one or more spiritual entities while en route, and have been re-transmitted by these entities until they have come into vibratory harmony with some human receiving station or life.

Benjamin F. Woodcox, Fragments of Spiritual Knowledge Pertaining to the Spiritual World, Woodcox & Fanner, Battle Creek, Michigan, 1923