Normally, all instances of X on an object have the same value at any given time.
October 4, 2019 · Throne of Eldraine
If an object gains an ability, the value of X within that ability is the value defined by that ability, or 0 if that ability doesn’t define a value of X. This is an exception to rule 107.3h. This may occur with ability-adding effects, text-changing effects, or copy effects.
May 3, 2019 · War of the Spark
If an object gains an ability, the value of X within that ability is the value defined by that ability, or 0 if that ability doesn’t define a value of X. This is an exception to rule 107.3h.
January 25, 2019 · Ravnica Allegiance
If an object gains an ability, the value of X within that ability is the value defined by that ability, or 0 if that ability doesn't define a value of X. This is an exception to rule 107.3h.
October 5, 2018 · Guilds of Ravnica
If an object gains an ability, the value of X within that ability is the value defined by that ability, or 0 if that ability doesn’t define a value of X. This is an exception to rule 107.3h.
January 23, 2015 · Fate Reforged
Some objects use the letter Y in addition to the letter X. Y follows the same rules as X.