= imei certificate = imei signature = imei key file.
CERT file is used to unlock the network, so that another sim card can be used with the phone or changing the IMEI number, country code... (all these are illegal in many countries) Most of all 2014/2015 year samsung phone have new security regarding imei. All phones out from factory with imei signature(key,cert) calculated in factory and stored in DB.
Qualcomm phones have more strong security and even not allow to read cert. Exynos and SPD phones allow read/write cert.