Sleeves are protective plastic covers used to prevent cards from getting bent or scratched. If a player uses sleeves, they must all be the same (including on the first vanguard) so their cards aren't marked. However, G units do not need to have the same sleeves as the cards in the main deck, which is recommended to differentiate them.
Each sleeve pack contains 53 to 60 copies of size 89x62mm. In Japan, a volume pack costs 500 yen (roughly 4 Dollars, tax included), while a limited edition costs 700 yen (roughly 6 dollars.)