(0) The default approach for connectable features is to connect them (Enums.LabelLineConnectionMinimizeLabels). This value corresponds to no value being specified for a LabelDefinition::lineConnection property.
(1) Line geometries with the same label and coincident end vertices should be considered together as a single, continuous geometry when placing labels. For example, if a single label is being placed on a line feature, then the single location will be chosen along the combined geometries. This combining will ignore junctions, so it may be ambiguous which line feature after a junction is the continuation of the earlier geometry.
(2) Keep one label per line feature geometry.
(3) Line geometries with the same label and coincident end vertices should be joined, until they hit a junction. For example, if a single label is being placed on a line geometry (composed of multiple features' geometries, but all having the same label text), then a label will be placed on both sides of any junction, to make it unambiguous which geometries are the continuation of the initial geometry.