An auction came up recently with a pickup partner. At the end of the auction, we were at the five-level, mercifully making when the opponents missed cashing an Ace. But, it need not have been that way.
After the auction was over, my pickup partner acknowledged knowing that a call was a cue in support but claimed to have no idea which suit I was supporting. This seemed odd to me, but I suppose a general rule of thumb is worth noting, again.
When one of two strains can be agreed below game easily but the other cannot, odd cues general support the inconvenient suit.
The example from the near disaster. After a minor opening and spade response, I jumped to 2NT, 18-19 balanced. Partner then rebid 3H naturally. I bid a minor next. As 3H was GF by agreement, 3S could agree spades with space to cue, but I could not support hearts as easily. Therefore, cues support hearts.
A second, related corollary.
When two suits can be supported easily, cues and other slam moves generally support the suit that is least economical to support.
This happens where, for example, partner could support spades at 2S or hearts at 3H in a GF sequence. Splinters in this situation support hearts, the least economic raise. With shortness and spade agreement, support spades and then splinter if you can.
Using this parallel, had my pickup partner used transfers, 3D would show hearts, and hence either suit could be supported below game. An immediate cue by me in that instance, using this rule, would support spades instead, as this would be the least economical raise. It is only by a sliver less economical, and arguably not even so. But, a rule of thumb applied even when it has no technical advantage has an advantage nonetheless of consistency and expectation.
A caveat, though.
When neither can be supported economically, flags probably apply.
Suppose that partner had longer spades and naturally bid 3S at rebid. Now, neither hearts nor spades could be supported below game. If I wanted to make slam noise in this event, I would ideally be able to use flags, with 4C super-accepting hearts and 4D super-accepting spades.