What is the explanation of the phrase "And the custom is established and transferred at once in menstruation and post-natal bleeding according to Abu Yusuf, and this is the fatwa, but according to them, repetition is necessary" for the Hanafis, in terms of meaning and language?

The Fatwa (definition)'s Summary:

The customary period (of menstruation or post-natal bleeding) is established and shifts with a single occurrence in menstruation and post-natal bleeding according to Abu Yusuf, and his view is adopted for issuing fatwas. Whereas, according to the two companions (Abu Hanifa and Muhammad), it requires repetition. The difference of opinion manifests in the ruling concerning a woman who experiences a departure from her custom once, and then the bleeding continues. According to Abu Yusuf, she reverts to the last duration she observed, while according to the two companions, she reverts to her old customary period. However, if she observes that (deviation) twice, and then the bleeding continues, then by consensus, she reverts to the duration she observed twice.