This is the fourth of the five terms, or bounds, into which the Egyptian table divides Libra. It runs from 21 degrees of the sign to 28, a stretch of 7 degrees held by Venus. It opens where Jupiter's term closes at 21 degrees, and it gives way to Mars's term at 28 degrees, so a planet moving forward through Libra leaves Jupiter's bound and enters Venus's at 21 degrees exactly.
Set out in degree order, the five terms of Libra run Saturn (0-6), Mercury (6-14), Jupiter (14-21), Venus (21-28) and Mars (28-30). This term of 7 degrees sits between the two extremes of the sign, where the widest is Mercury's at 8 and the narrowest is Mars's at 2. The unevenness is not an accident of copying. No sign in the Egyptian table divides its five terms equally, and no two signs divide them the same way, which is why the table has to be consulted rather than derived.
The five rulers are the five planets visible to the naked eye. The sun and moon take no terms at all in this system, so each sign is shared out among Saturn, Jupiter, Mars, Venus and Mercury, every one of them holding exactly one stretch of it. A planet standing anywhere between 21 and 28 degrees of Libra is standing in Venus's bound, and Hellenistic technique reads the lord of the bound from the degree occupied rather than from the sign as a whole. The table this atlas follows is the Egyptian one, transmitted by Vettius Valens in his Anthologies and recorded by Claudius Ptolemy in the Tetrabiblos, which stayed in ordinary use after Ptolemy set out his objections to it and offered boundaries of his own.