Form 5309 is used in conjunction with Form 5300 when applying for a determination letter as to a deferred compensation plan's qualification status under section 409 or 4975(e)(7) of the Internal Revenue Code. The information is used to determine whether the plan qualifies.
The latest form for Form 5309 - Application for Determination of Employee Stock Ownership Plan expires 2021-04-30 and can be found here.