四分律行事鈔資持記 2.1101

2.1101

四五道船二行。云下至者，示極小量；已上皆開，減應須制。疏云道船途路多有留難。

The fourth and fifth are road and boat travel. Saying "down to" indicates the minimal amount; above this all are permitted exceptions, while below it must be prohibited. The Commentary says that roads and boat routes have many delays and difficulties.