The segment formed by a Pipe Connection, and one of its immediate child connections. A segment is a visual representation of relationship between other connections. Each connection has an array of connection indices (starting at 0), and segments can be represented as a series of indices. For example, [0,1,0] would represent the first pipe, second connection, then first sub connection.
Consider the simple pipe below. Segment [0,0] is between the origin and lower left connection. Segment [0,0,0] is between the lower left and lower right connections.

We can add two new connections, which could look like the following.

Because of where we added our connections, both segment [0,0,0,0] and [0,0,0,1]. Since [0,0,0,1] was added later, it is the second child, hence the number 1.
Note that segment indices are represented in the Project Browser.
[
{
"name": "pipe",
"appearance": "auto",
"flanges": true,
"lineVariant": "solid",
"start": "none",
"end": "none",
"fill": "",
"stroke": "",
"width": 10,
"origin": {
"x": 258,
"y": 168,
"connections": [
{
"x": 258,
"y": 236,
"connections": [
{
"x": 307,
"y": 236,
"connections": [
{
"x": 360,
"y": 236
},
{
"x": 307,
"y": 168
}
]
}
]
}
]
}
}
]