What is considered a valid reference?

It should include the following:

In order to be credited the reference should be submitted to Recruitment Team via Referral Platform https://career.luxoft.com/referral/ before the start of recruitment process.

The reference will only be accepted and credited if within three months prior to the reference the referred person has neither been involved in the recruitment process nor has already been referred by someone else. Please be advised that the Company may refuse to admit the person you’ve referred to the interviewing process without further clarification of the reasons.