A more thorough explanation:

Res merae facultatis

Res merae facultatis is a legal term used in Scots law. It refers to a matter of mere power, which means it is a right that someone has but does not have to use. For example, a property owner has the right to build on their own property, but they do not have to exercise that right. Similarly, someone has the right to walk on the seashore or sail on a navigable river, but they do not have to do so. These rights are never lost by not being used for a long time because they can be used at any time.

• A property owner has the right to build on their own property, but they do not have to exercise that right. This is an example of res merae facultatis.
• Someone has the right to walk on the seashore or sail on a navigable river, but they do not have to do so. This is also an example of res merae facultatis.

These examples illustrate the definition of res merae facultatis because they show that these are rights that someone has but does not have to use. They can choose to exercise these rights at any time, but they are not required to do so. These rights are never lost by not being used for a long time because they are of their essential character that they may be used or exercised at any time.