SOPS, "A" days are basically long-call reserve but with no rules governing them. They are not considered duty but, the day before an "A" day is rostered the pilot must contact the company between noon and 2pm to check if there is a duty for him or her. If so, he or she has a minimum of 10 hours to report. If not, he or she must be immediately contactable on an "A" day between 0600 and 1400 and be able to report for duty within 10 hours of being notified.
Now, that "long-call reserve" is gone. All there is now is short-call, which is a duty and is covered by many rules as well as requiring being able to report for duty within 2 hours and 15 minutes of notification.
Reserve can start and end at any time of the day. Pilots accrue no pay for reserve unless they are rostered for more than 30 days of it per year.
Unlike normal airlines, where generally only the junior pilots do reserve and only reserve, at Cathay everyone does reserve, mixed in with regular flying.