Reversionary and contingent legacies

This is a really basic question - legacies aren't my specialism but am currently preparing some marketing materials, so hope you'll excuse me.

Is there a difference between reversionary and contingent legacies? My understanding is that a reversionary legacy is when the assets are used by someone else before going to the charity and that contingent is when the transferral of assets is dependent on another event happening. However, I could see circumstances when they could be one and the same thing - i.e once x has passed away the assets will be transferred to the charity - in my mind this could be either.

Are they generally seen as two separate things, or could I explain them as one and the same?

I'd be grateful for any clarification.

Thank you!

Sally