What causes Overwatch's Capture Highlight feature to be disabled in game?

Overwatch's new Capture Highlight feature allows you to press F9 to save your last 12 seconds of gameplay, or the active Play of the Game. After the game, you can go to your Highlights to replay what you saved or export it as a video file.

However, this feature is not available at all times. For example, if you try to use it in the Practice Range you'll see the error message "Highlights unavailable". I was also unpleasantly surprised when, after making an exciting play in a No Limits arcade game, I pressed F9 and nothing happened, not even an error message. It also doesn't seem to be supported in pre-game skirmishes.

When am I able to use the Capture Highlight feature? Is it only in particular game modes? In modes where it is supported, is it available through the entire game, or is it disabled at times the clock isn't running, such as the set-up phase and the few playable seconds after a round? What about during the Kill Cam, or Death Spectating? What if you're spectating a match instead of playing it?

(This is all from observation (as no official source exists) since the feature hit live a few months ago)

From what I've experienced using it in the tens of gameplay hours since this feature was added, Highlights is available during:

• Prepare phase (post character selection, before the doors open for attackers)
• Game phase (round started)
• Mid-round phase if started before the round's end (e.g.: you can capture a highlight which ends after the "Round complete" screen and have that screen appear in it)
• End phase (slow-motion "Victory"/"Defeat" screen, before the winning team is shown)

This applies to QP, Competitive, Custom Games (except as said below) and practice game modes like "Vs AI" (except for the Practice Range & Tutorial).

You can't use Highlights in these times:

• First 5-10 seconds of a game (highlight would've started during character select if you could)
• Waiting for players phase
• Skirmish, both in custom games in Skirmish mode and in "Skirmish while searching"
• If a server error occurred and the feature isn't working for this match (rarely happens)