What do issue-based and access-based actually mean?
Access-based subscriptions are time based subscriptions that give you access to everything but only for a specific amount of time. At the end of that time period, if your subscription is not renewed then you will lose access to all the paid content. For example like Netflix.
Cons: subscribers are forced to do single issue purchases to get the back issues and the back end system needs to ensure that the user is granted at least one previous issue when they subscribe or it is a terrible experience.
Cons: expired users may complain that they've lost access if they don't understand the model
Issue-based subscriptions give the user access to that issue only, but they retain access to it permanently e.g. if you were to buy access to a particular film on prime video, that purchase would allow to access that film for as long as you wanted.
Pros: new subs get the whole back catalogue
Pros: much simpler to understand and to implement
Cons: not good if you have good single issue sales of back issues
What are Third Party Auths and how do they work?
For Third Party Auth, it depends on what the back end system does. For some systems that we use a lot (for example the Adobe DPS Legacy Subs) we can configure it so that we do support either and can convert the response from one to the other. For most third party auths, however, it is one or the other. If you are unsure what model your third party backend supports you should check with them.
What about iTunes/Google Play subscriptions?
IAPs - In-App Purchases are extra content or subscriptions that you buy inside an app. These purchases are done through and managed by the Apple App Store or Google Play (depending on your device's operating system).
For iTunes, we can support either mode - it is a configuration setting.
For Google Play, we currently only support Access Based due to the nature of the Google Play receipts. We are currently working on implementing Issue Based however.
Note that for both of these it will ALWAYS be issue based in the case of Single Issue in app purchases