The purpose of this is to detect people who have binocular vision but correct one eye with a contact so they see distance with one eye and near vision with the other.
Item 17.b. Do You Ever Use Near Vision Contact Lens(es) While Flying?
The applicant should indicate whether near vision contact lens(es) is/are used while flying. If the applicant answers yes, the Examiner must counsel the applicant that use of contact
lens(es) for monovision correction is not allowed. The Examiner must note in Item 60 that this counseling has been given. Examples of unacceptable use include:
The use of a contact lens in one eye for near vision and in the other eye for distant vision (for example: pilots with myopia plus presbyopia).
The use of a contact lens in one eye for near vision and the use of no contact lens in the other eye (for example: pilots with presbyopia but no myopia)
Since this results - at least initially - in a substantial loss of depth perception - the FAA frowns on the practice.
Wearing contact lenses that simply correct you back to class 1 standards is fine.