Under 623 is classed information about the design, manufacture and technical handling of weapons and fighting equipment used on land, at sea, in the air or in space, whether by the armed forces or by civilians (e.g. for police work, hunting, shooting and competition).
The operational, tactical and strategic use of this equipment and the organization of personnel etc come under 355/359.
BS 1000[623]:1974 history
1974BS 1000[623]:1974 Universal Decimal Classification - English full edition - Military engineering