In training corrections officers, New Zealand combines local on-the-job training under institutional staff training officers with centralized backup courses, thus providing resources for standardization checks. Regarding release under supervision, probation is provided as a sanction which may be imposed by the courts instead of a prison sentence. Statutory probation is a mandatory part of any sentence of imprisonment of 1 year or more. Parole is provided as a release from custody during sentence, under which the offender is subject to recall at any time up to the expiration of the term of sentence. Remission of sentence after serving three-quarters of it is available to virtually all prisoners. With the exception of imates deemed to be dangerous, all prisoners become eligible at some point for home leave. Work release is also available under certain conditions. Opportunities for vocational training in New Zealand's prisons are very limited. Educational opportunity is the major voluntary source of vocational training. This is largely theoretical, since few work-related placements are available. The principal aims of inmate classification in New Zealand are to determine the level of security and custody required for the inmate and to place the inmate in an appropriate institution within that security level. Limitations upon classification are imposed by the structure of the prison system, overcrowding, and the complications associated with having to provide segregated accommodations for female inmates with babies, homosexuals, special medical cases, child molesters, former law enforcement and corrections officers, serious drug offenders and mentally ill inmates.