A subscription is an issue or volume publication that is added as a product. Since issues can be paper (in terms of a printed journal) or web access (where there is no physical mailing), issues are defined the same as revenue recognition periods. In membership, these periods are monthly, but in subscriptions, they can be as frequent as the user specifies and have controlled recognition dates.
An issue-based subscription can start and end on any issue. A volume-based subscription must start at the beginning of the volume and end with the issue at the end of the volume. For example, it must start January 1st and end December 31st for volume number 112.