A battlefield zone, as opposed to an instanced zone, is a zone that is shared among all players and parties currently in that server in that particular zone.

This means that on battlefield maps you may run into human players other than members of your own party.

The game may sometimes lag if a lot of other human players are nearby.

"Kill stealing" and "loot stealiing" can sometimes be an issue.

Tip: If several players need to kill a specific creature, or need to collect a specific loot item, they should team up in a party. This way, if one party member completes a quest objective, it will be completed for all nearby party members.

Other types of "griefing", especially "ganking" are mostly prevented by the fact that TR doesn't allow friendly fire in PvE areas.