This is the 5187th most frequent Russian word.
неофициальными
"Unofficial" or "informal".
неофициальными is used as an adjective in instrumental plural form, describing 'данные' (data), indicating their unofficial nature.
Эти данные считаются неофициальными и их точность не гарантируется.
These data are considered unofficial and their accuracy is not guaranteed.
неофициальными serves as an adjective in instrumental plural form, describing 'совещания' (meetings), showing their informal nature.
Совещания были неофициальными, поэтому строгого регламента не придерживались.
The meetings were informal, so strict regulations were not followed.
неофициальными modifies 'источниками' (sources) as an adjective in instrumental plural form, characterizing the sources as being unofficial.