Intelligibility is a measure of how well speech can be understood. The other way to produce a sound incorrectly is to omit it entirely.
Every language has rules about how sounds can be combined.
In this case, classes of sounds, rather than single sounds are affected. For example, the child may have their own rule for sounds produced at the back of the mouth - they make them all at the front of the mouth. Adults and children can have articulation problems. Not all languages have the same rules for combining sounds, nor do all languages have the same sounds. A child who is learning English as a second language must also learn a new sound system.