A ball is an endpoint of a leaf string. String only have two (2) endpoints, hence, there are only two balls. Yesegalo-wise, each ball is either a start or stop identifier. As an arbitrary ordered pair, they are referred to as abscissa (azure) and ordinate (yellow).
It's called a 'ball' (and sometimes 'dot') because as the string folds upon itself, the endpoints coordinate ("connecting the dots") into a loop, and the overall structure usually assumes a shape that is similar to some ovoid with boundary (cf., compact blob) in stereo.
Note (+): Balls are pseudo-theoretical (abstract) objects that only exist as placement functions. There is no 'ball', so-to-speak, just a representation of an endpoint (dot).