Is it a way to offer subscription as In-App purchase but for service and not for content? I want to sell my app as a service where customers could pay for using it for a period of time. Is it possible? Where could I read more about it?
Apple developer site offers a summary called: In-App Purchase for Developer. It contains an In-App purchase category, where the subscription is time limited:

Non-Renewing Subscriptions
Non-Renewing Subscription allow the sale of services with a limited duration. Non-Renewing Subscriptions must be used for In-App Purchases that offer time-based access to static content and are only available for both iOS and OS X apps.
Examples of Non-Renewing Subscription purchase types are:


One week subscription to voice guidance feature within a navigation
app
Annual subscription to online catalog of archived video or audio


Services
In-App Purchase also gives you a way to sell digital services within your application. Digital services are usually either Non-Consumable or should be a Non-Renewing Subscription. The differentiation would be whether access to the service is limited to a specific time period.