This question stems from looking at the rules on Critical Hits:
When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together [...]
[...] If the attack involves other damage dice, such as from the rogue’s Sneak Attack feature, you roll those dice twice as well.
Are Sneak Attack's dice part of "the attack's damage"?. An example where this matters is the Piercer feat, which states (emphasis mine):
[...] Once per turn, when you hit a creature with an attack that deals piercing damage, you can reroll one of the attack's damage dice, and you must use the new roll. [...]
So a specific example of my question could be this: Does the Piercer feat allow you to reroll a Sneak Attack damage die?