Out of the box the only way is to put the field ONLY on the transition screen where X will see it. I presume you are somehow restricting the transition to user X or just want the user executing the transition to edit the field. Out of the box, JIRA doesn't have field level permissions. The behaviours plugin may allow it.
Thanks for the response Joe.
Scenario:In the transition executed by X, there are field values which Y should not be able to edit though Y is the assignee. Also, X should be able to edit the fields even after the transition.
I don't see it happening without a plugnin. Removing field from the edit screen is not an option.
What you're describing there is summarised as "field level security"
JIRA doesn't do that.
There is a field level security add-on by Quisapps which is well worth a look.
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