Simultaneous resources consist of two or more resources scheduled with the same start date and time within a job operation. These resources have the same value in the Scheduled Seq field of the Routing window. Simultaneous resources must begin at the same time, but they may complete at different times depending on the Usage Rate or Amount.
Alternate resources consist of other resources-or groups of resources-used instead of the primary resource in the job operation. Alternate resources are defined on the Alternate Resources window. They are invoked in one of two ways:
During backward scheduling, when the use of alternate resources is the only means for a job to complete by the Requested Due Date
When a resource for an operation is unavailable
You can also use alternate resource sets. In infinite scheduling, you can change an alternate resource to a primary resource, This specifies it as a one-for-one substitution, or substitution by a group when the resources in the group are simultaneous. In constraint-based scheduling, you can specify the resources in a sequence, and have them span multiple schedule sequences. See: Alternate Resource Sets