The Absolute Value Card (AVC) is a function within a card; it causes some numerical value to be viewed as positive (or negative, with proper care) regardless of its sign (hence why it is named after the similar mathematics operation). While not incredibly important, this card archetype crops up in enough decks that it deserves its own entry.
There are a few forms that an AVC shows up in, and their usage can often vary.
The classical form is when you write down "+|1000| points!" to make sure that Munchkin Billy can't make all point values negative so that he wins with a perfect 0 on his score. This can also be used to re-enforce the threat of a mega point card.
A less common form is to make sure that counting numbers stay positive, especially when the results would be nonsensical otherwise. Examples include drawing cards, discarding cards, and taking extra turns. Your playgroup might reach a consensus on what "Draw -1 cards" means, however there are other quantities that would be less clear (such as creating a negative amount of tokens, especially when you have less than the number of tokens that would need to be negatively created.)
Related to Points, it's not uncommon to see "+|10| Mana!" or the like. This is usually to prevent damage done by sign changing, particularly if a non-physical resource is accumulated over multiple rounds.
Like with Usage, there are a few specifics to add layers of fortification to the Absolute Value function.
The classical method is to simply wrap the number in the AV function. There is potential for abuse here though, if a played card changes all plus signs into minus signs, this will do nothing. Changing "+|1000| points!" to "-|1000| points!" would be the result of this sign change, which causes you to lose 1000 points instead of gaining those points.
A more experienced player, after seeing the failings of the Classical implementation, will go fore a more technical (and more wordy) method. This method is to use words to describe what you want. An example would be "Gain |1000| Points." It would take more creativity to defeat this, but it is still possible, such as a card that says "On all other cards, change Gain to Lose."
The more discerning player will see the downfalls of poorly worded cards, and try to either brute force their way through the problem ("+1000 Points! This card cannot be altered, nor can it be affected by any other cards, even if those card's abilities are unstoppable.") or to put thought and consideration in how they word it ("You gain 1000 points. The number of points you gain from this card cannot be altered.") This high level of thinking and play is usually reserved for competitive play, or for players who carry jargon from other games, such as Magic.