Application form - "acked"

Throughout the Application Form module code I see references to variable names containing the term "acked", such as:
$acked_str,
$acked_link,
$acked_time

And:
$submit->acked,
$submit->acked_time and
$submit->acked_user

What does "acked" mean?

Then there is the similar name "oacked". What is "oacked"? And how does to differ to "acked"?

Thanks.