In general, the first execution host satisfies certain resource requirements that might not be present on other available hosts.
By default, LSF selects the first execution host dynamically according to the resource availability and host load for a parallel job. Alternatively, you can specify one or more first execution host candidates so that LSF selects one of the candidates as the first execution host.
When a first execution host is specified to run the first task of a parallel application, LSF does not include the first execution host or host group in a job resize allocation request.