Implementation

An implementation is code, program, software, or even business operation that acts in accordance to a specification.

For example, Pixelfed and Mastdon are implementations of several specifications that enables it to function properly within a large subset of the Fediverse.

Had neither of those software been implemented properly in accordance to any specification, then neither of them would have been able to interact with each other, for example, neither of them would have been able to follow accounts, from each other.