SLP

For information about the SLP exams (1s to 4s), as well as sample questions and answers, you can consult the following site:
SLP exam levels

An extensive glossary of military terms can be found at:
nato-bilc trillingual

The Spanish Ministry of Defence also recognises other exams (e.g. Cambridge, Trinity, etc.). In order to know what results you need to obtain in these exams to reach a certain level according to Stanag, you can consult the Boletín Oficial del Ministerio de Defensa.
Regarding the Cambridge exams, the BOD (Miércoles, 27 de mayo de 2020) says:
University of Cambridge-ESOL Examinations (Cambridge-ESOL): Tanto el certificado de C1 Advanced (CAE) grado A, B o C como el certificado de C2 Proficiency (CPE) grado A, B o C, acreditarán un SLP 3333.
B2 First (FCE) grado A, B o C, acreditará un SLP 2222.

B1 Preliminary (PET) «Pass», «Merit» o «Distinction», acreditará un SLP 1111.

And with regards to Trinity College London: Integrated Skills in English (ISE):

ISE III, e ISE IV «pass», «pass with merit» o «pass with distinction», acreditarán un SLP 3333.

ISE II «pass», «pass with merit» o «pass with distinction», acreditará un SLP 2222.

ISE I «pass», «pass with merit» o «pass with distinction», acreditará un SLP 1111.

*If you would like to see what the FCE exam is like, you can download a sample paper by clicking on the following link (you only have to click on one 'see sample paper' link):
first certificate