It might be a while after you ask this question, however I believe the answer to your question is as below
Click 'Disable' to hide an option so that it is no longer available for selection. Options that have been used cannot be removed (to preserve data integrity), but due to changing business requirements, they may become invalid over time and so you may wish to make them unavailable for new issues
So in simple, it disables the custom field without purging the custom field properties and data from database.
Hope this answer your question, cheers :)
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