Basic EMC publications
As their name implies, the IEC's Basic EMC publications specify the general conditions or rules necessary for achieving electromagnetic compatibility. They serve also as building blocks for the IEC technical committees that develop EMC Product standards.
Some Basic publications are normative International Standards while others are non-normative Technical Reports. These standards are contained in the IEC 61000 or CISPR 16 series of publications.
The compatibility level is clearly a key quantity when it comes to setting limits. The IEC defines it as "the specified electromagnetic disturbance level used as a reference level for co-ordination in the setting of emission and immunity limits."
By convention, the compatibility level is chosen so that there is only a small probability that it will be exceeded by the actual disturbance level. The probability distribution depends entirely on the method used for evaluating the levels (samples of time, location, intervals, etc.) but frequently the 95% probability level is defined as compatibility level.