Two quite different practices go under this name and the difference is worth stating first, because most confusion about the subject comes from running them together.
The older practice uses fixed seals. Grimoires supply a specific figure for each spirit, and the figure is treated as belonging to that spirit in the way a signature belongs to a person; it is drawn in order to summon, bind or constrain, and it may not be altered. Planetary characters and angelic seals work the same way, and some are derived by tracing a name across a magic square, which is a construction method but not an invention: the result is determined by the name and the square.
The modern practice inverts that. Austin Osman Spare, in the early twentieth century, took a statement of intent written in words, struck out the repeated letters, and combined what remained into a single compressed figure with no readable content. The figure is then charged in a state of concentration or exhaustion and, crucially, forgotten, so that the intention can operate without the conscious mind interfering. Nothing is being summoned; the target is the practitioner.
Chaos magic adopted this as its central technique in the 1970s, and it spread from there into general use, including into internet culture, where the word sigil is now applied to images made and shared with no ritual attached at all.
The atlas keeps both, and treats the seal of a named spirit as a different object from a made sigil.