In Arbor, we do not have a stand-alone AM or PM attendance column that you can update. Arbor calculates a student's AM/PM roll call mark based on the roll call times that you have set up. We have steps in the guide here on how you can edit your roll call times in Arbor.
Your previous MIS might have had the ability to update an AM/PM roll call mark via an Edit Marks page without it being linked to or affecting the lesson mark they have received in a register, like this:
In Arbor, this works slightly differently. Taking a look at the diagram below, the student will have automatically been given their morning mark based on their lesson 3 mark.
This is because the AM Roll Call time has been defined between 08:00 and 10:00 and the student received a present mark in the register within this time and this was used as the best mark.
So without having to manually update or check the AM column, the student will automatically receive their morning mark as they were marked present at 09:30 in their lesson register. It is possible to edit the marks contributing to the Roll Call mark using our Roll Call Marks page.
It's also important to note that here at Arbor we cannot advise on the times to set your roll calls up with, however, the DfE has issued the following advice in their School Attendance Guidance here (page 10): "Schools should have a policy on how long registers should be kept open; this should be for a reasonable length of time but not that registers are to be kept open for the whole session. A pupil arriving after the register has closed should be marked absent with code U, or with another absence code if that is more appropriate."