What it is
A NATO agreement, not a specification
STANAG 4737 is the NATO Standardization Agreement covering "UAS weapons integration": interoperability for putting a weapon payload onto an unmanned aircraft system (UAS). Like every STANAG, it is not itself the technical specification. It is the agreement by which NATO nations commit to implement one, here AEP-82, Edition A, the Allied Engineering Publication that carries the detailed technical requirements. Participating nations "agree to implement the following standard": AEP-82. Ratification and national implementation are the steps that give the STANAG force, and a nation can ratify with reservations; the document itself says "the partner nations are invited to adopt this STANAG" rather than being bound to it. So whether STANAG 4737 is mandatory has no general answer, only a contractual one: it binds the nations that ratify it, and it reaches a contractor or integrator when a tender or contract names STANAG 4737 or AEP-82.
The interoperability requirements it responds to
The STANAG's stated aim is to respond to three requirements: to allow a single weapon type to be flown on different UAS platforms, to standardize the interface between the UAS Control System and the unmanned weapon platform, and to control the targeting and launch of weapons from UAS Control Systems that are not built alike. Two other STANAGs are named as "cited mandatory" alongside this one: STANAG 4586 (the UAS Control System interface standard, carried in AEP-84) and STANAG 7085 (interoperable data links for ISR systems). Together, the document says, they are meant to produce standardized Information Exchange Requirements and a common data format for targeting and releasing the weapon payload.
What the guidelines cover, and what they do not
The guidelines address integrating a weapon payload into a UAS so as to meet "the applicable weapon system safety requirements," and to let a conformant UAS Control System target and release a weapon from a conformant aircraft. They also anticipate weapons built to the emerging NATO Universal Armaments Interface (NUAI). What they do not cover is a nation's complete safety case: the document is explicit that its guidelines reach only "top level" safety requirements, and that individual nations may have their own additional national safety requirements on top. A programme working against this STANAG should expect those national additions to be documented separately, not folded into it.
Reporting and review
Nations report their effective implementation using the form in Annex H to AAP-03, Edition J; partner nations report adoption using Annex G of the same publication. The STANAG itself is reviewed at least once every three years, with proposed changes routed through the tasking authority: the NATO Naval Armaments Group (NNAG) and the Joint Capability Group on Unmanned Aircraft Systems (JCGUAS).