Example when being able to determine that order matters: periapt of wound closure, an attunement item that benefits spending hit dice.
Picture the following scenario: The party is beat-up after a few fights and decide to take a short rest. There is a brief argument as to who needs healing most, and eventually, one character is chosen – let's call him Genji.
Thus, Genji pulls out the party's periapt of wound closure, and spends the next short rest fiddling with it. Now, at the end of the short rest, two effects happen:
A character can spend one or more hit dice at the end of a short rest, up to the character’s maximum number of hit dice, which is equal to the character’s level.
And
Attuning to an item requires a creature to spend a short rest focused on only that item while being in physical contact with it (this can’t be the same short rest used to learn the item’s properties). This focus can take the form of weapon practice (for a weapon), meditation (for a wondrous item), or some other appropriate activity. (…) at the end of the short rest, the creature gains an intuitive understanding of how to activate any magical properties of the item, including any necessary command words.
Obviously, it's highly desirable for Genji to first attune to the magic item, then spend hit dice, since their rolls will be doubled – but is that actually viable?
Best Answer
RAW, it appears to work.
There is no guidance given for the order in which these two effects would occur. Both happen "at the end of a short rest". So when it comes time to roll hit dice at the end of the short rest, we can ask "am I attuned to the periapt?", and the answer will be "yes", because it is the end of the short rest.
However, I would not allow this to work.
At the table, I would rule contrary to the ruling I offered above. The narrative idea behind a short rest is that you gradually recover over the course of the hour, and the narrative idea of the periapt is that it is supplementing that recovery for the duration of your rest. Within the fiction of the game, you do not suddenly pop up to being healthy at the end of the hour, and in the same way, without the periapt supplementing your recovery for the duration of the rest, you should not benefit from it at the end of the rest.