Distance
Releases the zone behind after travelling a fixed distance into the next one: the general case, with release at the start and release at the end as its two extremes.
Zone sizes vary from link to link, and junctions are usually treated as points with no length at all, so one fixed distance will inevitably meet zones shorter than itself. Where it does, the release happens at the far end of that zone, and where the zone has no length the release is immediate. Both follow from reading the distance as "after travelling this far, or on leaving the zone, whichever comes first", which keeps the behaviour monotone in the distance asked for. Refusing instead would make the rule unusable on any network that mixes zone sizes.
Parameters
how far into the entered zone, greater than zero. A distance at or beyond the entered zone's length is rejected when the rule is consulted, because it would describe a release that happens after the transporter has already left that zone.