A khipu is built from a thick primary cord with pendant cords hanging from it, subsidiary cords hanging from those, and occasionally a top cord running the other way. Everything about that assembly can carry information. The knots are the obvious part: their type, their number and their position along a cord encode numbers in a decimal positional system, with the position furthest from the primary cord holding the units, and that much can be read reliably today.
Less obvious, and more interesting, is everything else. Cord colour varies and is combined by plying two colours together. Spin direction varies, S or Z, and so does the direction of the final ply, and these are choices a spinner makes deliberately and a reader can feel as well as see. Attachment can be recto or verso. Fibre is cotton or camelid hair. Each of these is a binary or small-set distinction available at every point in the structure.
Gary Urton has argued that the sheer number of independent binary choices available in a khipu gives it an information capacity far beyond what numbers require, and that the surplus is the argument for something more than accounting. What that something is, and whether it is language, is a separate question from whether the capacity exists.