Your tab condition is
p_web.gsv('MAI:SizeLimit')=1
however doing a lookup on the record (by typing the code, or doing an auto-complete) does not move that record into the session queue. (the browse-as-lookup procedure does, but by accident, not design.)
In other words, if you want to load the record of the related file into the session queue then you will need to add the embed code to do that.
In the ALI:MailBoxNumber routine, "After Validate New Value" embed
Access:MailBoxes.Open()
Access:MailBoxes.UseFile()
MAI:MailBoxNumber = ALI:MailBoxNumber
Access:MailBoxes.Fetch(MAI:PrimaryKey)
p_web.FileToSessionQueue(MailBoxes)
Access:MailBoxes.Close()
Cheers
Bruce