Fields are designated geographical areas to which the Head Worker of an area assigns a pair of same sex Workers under his supervision. One worker, usually the elder, is designated as the senior Worker in the pair.

Fields are reassigned periodically, usually once a year, with some workers moving to another field, while others may stay in the same field.

Workers are not to minister in the Field assigned to another without express permission. Perhaps due to this scripture.
Paul wrote in Romans 15:20:  Yea, so have I strived to preach the gospel, not where Christ was named, lest I should build upon another man’s foundation: