Hereafter are the definitions of reliability and credibility according to STANAG 2511 (January 2003, Intelligence reports, NATO Unclassified).
@Anne-Laure: I can’t find the STANAG 2022…
2. Reliability of the source is designated by a letter between A and F signifying various degrees of confidence as indicated in Table I below.
TABLE I - RELIABILITY OF SOURCE
A. Completely reliable D. Not usually reliable
B. Usually reliable E. Unreliable
C. Fairly reliable F. Reliability cannot be judged
3. Completely reliable (A). Refers to a tried and trusted source which can be depended upon with confidence.
4. Usually reliable (B). Refers to a source which has been successful in the past but for which there is still some element of doubt in a particular case.
5. Fairly reliable (C). Refers to a source which has occasionally been used in the past
and upon which some degree of confidence can be based.
6. Not usually reliable (D). Refers to a source which has been used in the past but has proved more often than not unreliable.
7. Unreliable (E). Refers to a source which has been used in the past and has proved unworthy of any confidence.
8. Reliability cannot be judged (F). Refers to a source which has not been used in the past.
9. Credibility of information is designated by a numeral between 1 and 6 signifying varying degrees of confidence as indicated in Table II.
TABLE II - CREDIBILITY OF INFORMATION
1. Confirmed by other sources 4. Doubtful
2. Probably true 5. Improbable
3. Possibly true 6. Truth cannot be judged
10. Confirmed by other sources (1). If it can be stated with certainty that the reported
information originates from another source than the already existing information on the same subject, it is classified as "confirmed by other sources" and is rated "1".
11. Probably true (2). If the independence of the source of any item or information cannot be guaranteed, but if, from the quantity and quality of previous reports its likelihood is nevertheless regarded as sufficiently established, then the information should be classified as "probably true" and given a rating of "2".
12. Possibly true (3). If, despite there being insufficient confirmation to establish any higher degree of likelihood, a freshly reported item of information does not conflict with the previously reported behaviour pattern of the target, the item my be classified as "possibly true" and given a rating of "3".
13. Doubtful (4). An item of information which tends to conflict with the previously
reported or established behaviour pattern of an intelligence target should be classified as "doubtful" and given a rating of "4".
14. Improbable (5). An item of information which positively contradicts previously reported information or conflicts with the established behaviour pattern of an intelligence target in a marked degree should be classified as "improbable" and given a rating of "5".
15. Truth cannot be judged (6). Any freshly reported item of information which provides no basis for comparison with any known behaviour pattern of a target must be classified as "truth cannot be judged" and given a rating of "6". Such a rating should be given only when the accurate use of higher rating is impossible.
Reliability and credibility, the two aspects of evaluation, must be considered
independently of each other. The resultant rating will be expressed in whatever combination or letter and number is appropriate. Thus information received from a "usually reliable" source which is adjusted as "probably true" will be rated as "B2". Information from the same source of which the "truth cannot be judged" will be rated as "B6".