The name of the maps like "X-1", "X-2", "X-3", etc. can be characterized as a general name, which refers to the cards of the same level.
In order to make it clear what are the cards with different designations:
X-1 maps are first-level cards that have the common name U-1, R-1, and E-1, depending on the type of card.
X-2 maps are second-level cards that have the common name U-2, R-2, and E-2, depending on the card type.
X-3 maps are third-level cards that have the common name U-3, R-3, and E-3, depending on the card type.
J-XX maps are third-level cards that share the name J-VO, J-SO, and J-VS, depending on the card type.
X-5 maps are fifth-level cards that have the common name U-5, R-5, and E-5, depending on the card type.
X-6 maps are six-level cards that share the name U-6, R-6, and E-6, depending on the card type.
X-7 maps are seventh-level cards that have the common name U-7, R-7, and E-7, depending on the card type.