It would be for the member to say that they were absent and this was public or official business, that they were following up on matters in their constituency.
There is, obviously, an expectation that members will try to organize their affairs so as to be able to be in the House. That's really part of the role of members, to balance those two things: the obligations in the constituency and the obligations in the House.
The act recognizes there will be times when the members cannot be in the House and it's going to be because they're engaged in other public or official business.