interrupter vs non-defining relative clause

[Xyz]

1) Are both interrupters and non-defining relative clauses same or different?

2) Kind was the epithet most often used to describe the world famous doctor.
Why the sentence is not like this?
Kind was the epithet that is most often used to describe the world famous doctor. or
Kind was the epithet which is most often used to describe the world famous doctor.

[Esl_Expert]

Hello Xyz,

1. I'm not sure what you mean by 'interrupters', but I assume you are referring to the commas that set off non-defining relative clauses from the rest of the sentence. The two are not the same. Commas are punctuation marks. Clauses are groups of words.

2. The words 'that was' (or 'which was') have been elided in the sentence. It is quite common to omit the relative pronoun in defining relative clauses. In this case, the word 'was' would be a repetition, so both the relative pronoun and the verb 'was' have been omitted. If you add the relative 'that' to this particular sentence, you also have to add the word 'was' to ensure that the verb form (used) is not changed to an active form.
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"People who say it cannot be done should not interrupt those who are doing it." ~ George Bernard Shaw

[Xyz]

1. Kind was the epithet most often used to describe the world famous doctor.
2. Kind was the most often used epithet to describe the world famous doctor.

3. Are both interchangeable?
4. In the first sentence, is 'most often used' a participle phrase?