Conjunction is the operation on which the imagery of the whole art turns, and it is also a plain description of putting two things back together.
What the word describes
Coniunctio means the recombination of what separation divided, the joining of two prepared substances into a third that is neither. On the bench that is a mixture, an amalgamation or a reaction. In the illustrated treatises it is drawn as a marriage, and the king and queen of the alchemical woodcuts are this operation in emblem form.
The emblematic register is not decoration added afterwards. Many of the surviving treatises are emblematic first and operational second, and the two readings ran together for the people who wrote them.
Where the number seven comes from
Exactly seven numbered operations, one per planet, is a modern arrangement rather than a historical one. The sources give different counts: Ripley's fifteenth-century Compound of Alchymy sets out twelve gates, and other authors give other lists in which these operations appear in other orders and alongside operations this scheme omits.
The atlas records the seven-step sequence because it is the one a reader will meet today, and records here that the tradition behind it was considerably less uniform than the numbering suggests.