When encrypting a file to send to a collaborator, I see this message:

gpg: using subkey XXXX instead of primary key YYYY

Why would that be? I've noticed that when they send me an encrypted file, it also appears to be encrypted towards my subkey instead of my primary key. For me, this doesn't appear to be a problem; gpg (1.4.x, macosx) just handles it & moves on. But for them, with their automated tool setup, this seems to be an issue, and they've requested that I be sure to use their primary key.

I've tried to do some reading, and I have the Michael Lucas's "GPG & PGP" book on order, but I'm not seeing why there's this distinction. I have read that the key used for signing and the key used for encryption would be different, but I assumed that was about public vs private keys at first.

In case it was a trust/validation issue, I went through the process of comparing fingerprints and verifying, yes, I trust this key. While I was doing that, I noticed the primary & subkeys had different "usage" notes:

primary:  usage: SCA
subkey:   usage: E

"E" seems likely to mean "Encryption". But, I haven't been able to find any documentation on this. Moreover, my collaborator has been using these tools & techniques for some years now, so why would this only be a problem for me?