After the verb kram the ātmanepada is optionally used, when it is not preceded by any preposition.,
The root 'kram' may always be conjugated as Atmanepadi when not having any upasarga. The upasarga will be defined in 1.4.59. The option allowed by this sutra is an example of what is technically known as aprapta vibhasa viz., an option which is not an alternative limitation to a general rule already found or known.
Examples: क्रमते or क्रामति 'he goes over.' Why do we say 'when it is without any preposition ?' For no option is allowed when it takes preposition. As संक्रामति.,