The difference is in timing. Postponement always happens before the starting signal with AP. Abandonment always after the race has started. Postponement is signaled with AP, which is sometimes accompanied with sign flag H or A:
Even if you can't see the flags, you can count the sound signals. Postponement is always signaled with two sounds, abandonment with three sounds.
If you have a hard time remembering which was again the H or the A flag:
- H (for home) is red like a traffic light which means stop, but expect a different signal later (ashore).
- A (for abandoned) is blue, for the disappointment that there will be no more racing (today)
In addition to sign flags A or H, AP ashore can be hoisted with a numeral pennant, whereby the meaning of the pennant- i.e. 1, 2, - till 6 - will signal the number of hours a race is postponed from the scheduled starting time. So if first pennant number one is used and after an hour exchanged for pennant two, the postponement is extended with one more hour till it is two hours after the original scheduled time. If there's no numeral pennant the postponement is for an indefinite time.
There are some interesting nicknames for AP. Well at least in my language. Translated we call this flag 'the rattail'. How about in yours? If there are any names you like to share, please leave them in the comments.