Generally, the delegation of ATS is in the interest of the service provider.  The reason for delegation of ATS is in many cases lack of airspace to provide a proper service to the users.
Activities involving inter alia cost-sharing or revenue sharing are subject to bilateral negotiations. Reasons for doing it could be manifold.
Provisions of this article are only applicable in the relationship between the Contracting States and do not constitute rights or obligations for third parties.
Edition: 2.0  Released Issue Annex 6A – Page 6-15
Article 6
Licensing and Training
(1)  The Contracting States agree that:
(a)  
an air traffic controller licence issued by one Contracting State, or

(b)  
an authorisation by a service provider, or

(c)  
an authorisation to a unlicensed State employee to operate as an air traffic controller,

is valid for the provision of air traffic services in the portion of the airspace of the other Contracting State within which the responsibility for the provision of ATS is delegated.
(2)   Training of ATS personnel of one Contracting State, providing ATS in the portion of airspace of the other Contracting State, shall include the requirements pertaining to the airspace concerned.
Article 7
State Aircraft
(1)  
State Aircraft other than those of the Delegating State may not enter that portion of airspace where the responsibility for the provision of ATS has been delegated without prior Diplomatic Clearance or special permission from the Delegating State.

(2)  
For State aircraft operating as GAT the same rules and procedures are to be applied as for Civil Air Traffic, but where necessary, special procedures should be established to permit their access to the airspace. OAT shall be subject to prior co-ordination between the military unit and the ATS Unit/Authority concerned.

This is actual practice which is applied by several ATS Units/Authorities in the world.
Different airspace classifications, restricted and reserved areas and/or special regulations in the neighbouring State including military procedures should be taken into account.
Generally a distinction between GAT and OAT is made to reflect their different objectives. For GAT, the relevant ICAO regulations (transferred into national law) would be applicable.
Page 6-16 – Annex 6A Released Issue  Edition: 2.0
Article 8
Co-ordination and Contingency Procedures for Military and Other Reasons
(1)  
Letters of Agreement (LoA) shall be supplemented by co-ordination and contingency procedures established by the Units/Authorities concerned.

(2)  
The ATS Unit/Authority of the Providing State shall provide the appropriate military Authorities/Units of the Delegating State with pertinent flight plans and other data concerning the flights in the airspace where the responsibility for the provision of ATS has been delegated.

Article 9
Publication
Contracting States shall agree upon the content of, and publish all relevant information regarding the portion of airspace, where the responsibility for the provision of ATS has been delegated, in their Aeronautical Information Package as defined in Annex 15 to the Chicago Convention.
The co-ordination and contingency procedures could include the following item, in accordance with Annex 11 and 2 of the Chicago Convention:
service the aircraft in the event of an  emergency steps to be taken in-flight contingencies steps to be taken to assist  the strayed  aircraft
steps to be taken concerning the interception of ( civil ) aircraft / unlawful use of the airspace
co-ordination between military authorities and air traffic services
co-ordination of activities potentially hazardous to civil aircraft
information exchange between the appropriate civil and military authorities / units
　
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