What is meant by the underlined part
There is no equivalence between the ranks of the defence forces and the police forces since there is no government established relativity in terms of rank. (source)
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The phrase 'relativity in terms of rank' could also be read as 'correspondence in terms of rank' or 'equivalence in terms of rank'.
There is no correspondence between, for example, a Commandant in one organization and a Major in another organization. Neither outranks the other, because there is no relative ranking between the two organizations - even though they might use the same words for their ranks.
To use an absurd example, there is no relativity in rank between the Chief of Surgery in a hospital and the Assistant Chef in a restaurant. Their organizations have nothing to do with each other.