English term (edited): index accident

"index accident" seems to be used in British English, in the UK legal system. It is legalese for "the accident we are talking about in this claim", or "the accident which is/was litigated", so basically an accident that was followed by legal action.

So, I cannot answer your question where you ask "He had two accidents. one 10 years ago and the other 2 years ago. which accident are they referring to.", because I don't know what the current claim/lawsuit is about. If you are sure that it is referring to one of these past two accidents, and there is no new, current accident (which would be the third), then you have to look at the context and deduce the answer from that. The rest of the text should make it clear which accident they are blaming for the conditions of the claimant, that they think are the consequence of said accident.

See "index accident" used in this sense:
https://www.johnmichaelporter.com/content-of-a-personal-inju...
"Content of a Personal Injury Report
...
An account of the client's present situation is given, as described by the client. Each symptom is described in a separate numbered paragraph. Information is given about the consequences of the index accident on the client’s work, hobbies and home life.
...
An opinion is given as to whether the results of the index accident will affect the client's overall general health, in the future.
...
An opinion is given as to whether the results of the index accident will affect the client's ability to obtain and retain work in the future."

https://www.hughkochassociates.co.uk/article/can-litigation-...
"Can Litigation Stress feel worse than the index accident?
...
In previous articles, HKA associates have looked at how litigation stress can affect recovery following a traumatic or stressful index accident (Koch and Elson 2008, 2009). Developing these themes further, several issues emerged in one particular case in which a claimant noted that ‘the litigation process felt worse that the actual accident’:"