Why do transition metals often have more than one possible ion, and why we distinguish these ions in the form of roman numerals in brackets?

They have variable Oxidation States, meaning they can undergo reduction to oxidise some other species or they can undergo oxidation to reduce some other species.
To my understanding, for A-levels, they can change oxidation states as they lose the s-orbital electrons before losing d-orbital electrons and the 5 d-orbitals are not all at the same energy.
With regards to using roman numerals, I believe it's just traditional convention.

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