Perhaps it's just my own ignorance but the Cossacks seem an especially nebulous and enigmatic topic.
The term is a little bit confusing because its took on ethnic connotations without losing its original meaning which was a settler granted frontier land in return for assuming military obligations. If employed in a military context, the latter meaning is probably implied. In other contexts, it probably refers to ethnic identities that emerged as a result of this policy (which has been resumed in the present day in the Russian Far East).