The first decan of Capricorn runs from 0 to 10 degrees of the sign and is ruled, in the scheme this atlas records, by Jupiter.
The card on the decan
In the Golden Dawn's assignment this decan carries the Two of Coins. The mapping is one to one and exhaustive: the thirty-six small cards numbered two to ten across the four suits match the thirty-six decans exactly, taken in order from the Two of Wands at the first decan of Aries. The suit follows the sign's element, so Capricorn is a Earth sign and takes a card of Coins, and the rank follows the position in the sequence, which puts the two here.
Two ruler schemes, and which one this is
Two schemes for decan rulership are in circulation and they disagree. The one recorded here runs the seven planets in Chaldean order continuously round the circle. The other, the triplicity scheme, gives a sign's three decans to the rulers of the three signs of its own element in order, so the rulers stay inside the element and restart at every sign. Both are attested in the tradition, neither is a corruption of the other, and a reader comparing two books will find them differing here without either being wrong.
What the atlas is recording
The degrees and the ruler are the attributions this atlas holds for this decan, and the card is the Golden Dawn's. All of them are traditions with variants rather than settled facts, and the decan itself, as a ten degree segment of a sign, is the only part that is simply arithmetic.