Includes personnel who were lost in circumstances or in areas that they may reasonably be expected to be known by the enemy.
Includes personnel whose loss incident is such that it is doubtful that the enemy would have knowledge of the specific individuals such as aircrews lost over water.
Unknown Knowledge. This means that the individuals whose time and place of incident are unknown. For instance if a plane went down with its crew members at an unknown location or ground personnel that was separated from his unit at an unknown time or place.
Also included in category 4 are those who do not meet criteria of categories 1 through 3. Those categories are found on Raymond Wagners Page at this site.
This category would mean their disappearance would be unrelated to degree of enemy knowledge. Technically, the individuals whose remains have been determined to be nonrecoverable as outlined in Department of the Army Technical Manual 10-286, January 1964, section 39.