Products on a system have relationships, dependencies, and conflicts between each other. Likewise, subscriptions have relationships that parallel the relationships of the software it represents. Some subscriptions allow virtual guests, some require other subscriptions, some conflict with other subscriptions.

Subscription define the relationships between installed products and each other and the systems on which those products are installed. Likewise, subscriptions can also define relationships between systems and how they interact within an environment. This is particularly apparent with virtual environments, where subscriptions can define different relationships for physical hosts and virtual guests, but there are other ways that systems can interact, such as data centers and cloud infrastructures. Subscriptions are a part of those meta relationships.

Using subscriptions to define these relationships introduces a lot of flexibility in how products and systems interact:

Part of the subscription service inventory is keeping track of subscriptions – not just what subscriptions are purchased but how many of those subscriptions are available.

When a subscription is first purchased, it defines the quantity of times that the subscription can be used. The subscription count is based on a certain element of the underlying system, most commonly its socket count (but it can be something else, such as the number of cores, depending on the specific subscription). The element of a system or software which is directly covered by a subscription is called an instance.

For example, for the subscription for Red Hat Enterprise Linux for 2 sockets, the product is Red Hat Enterprise Linux and the attribute is a physical socket pair. The socket pair is the instance.

A single subscription quantity is usually tied to a single socket pair (or other attribute). A system with eight sockets, then, requires more subscription quantities to cover its socket count than a four socket system. (This is called stacking.)

This simplistic arrangement, however, does not apply to all subscriptions.

Starting in October 2013, Red Hat began introducing other types of subscription relationships, such as:

Additionally, the 2013 subscription changes altered how virtual guests are handled in subscriptions. There used to be subscriptions for physical systems and then different subscriptions for virtual guests. In the current subscription model, the same subscription is used for both physical and virtual systems – but the quantity used is different, depending on whether it is a physical system or a virtual one.

As stated previously, a single subscription quantity is used per socket pair on a physical system. A virtual guest counts as a single socket, not a socket pair – so it is essentially half of a subscription quantity. When virtual guests are added to the inventory, the total number of available subscriptions is multiplied by two (the instance multiplier). This allows the subscription count to stay in whole numbers, even with virtual guests taking only a “half” quantity.

However, with some subscriptions counts multiplied by two; data center virtual guests not consuming any individual subscriptions; some subscriptions (Cloud Infrastructure) relating to multiple products installed on different systems; and older, pre-2013-style subscriptions all in the same environment — the actual counts listed in the subscription utilization pages or subscription management tools may not appear to reflect the quantities purchased in the contract. The fundamental counts are the same; most of the differences reflect changes to keep the count whole or new, more flexible subscription types.