Currently, The upstream job may allocate multiple workspaces, by calling multiple times the exwsAllocate step. In this case, when this run is selected in the downstream job, the exwsAllocate step from the downstream job will choose the first workspace that was allocated in the upstream job.
e.g.

Upstream Pipeline

// upstream job allocates workspaces from two different Disk Pools
def extWorkspace1 = exwsAllocate 'diskpool1'
def extWorkspace2 = exwsAllocate 'diskpool2'
// ...

Downstream Pipeline

def run = selectRun 'upstream-job-name'
// the downstream job will choose the first allocated workspace from the upstream job
def extWorkspace = exwsAllocate selectedRun: run

This feature allows the user to select a specific workspace that was allocated in the upstream job. For this, an extra parameter may be needed for the exwsAllocate step.
Each allocated workspace has its own auto-generated unique id, so we can make use of that.
e.g.

def extWorkspace = exwsAllocate selectedRun: run, workspaceId: 'unique-workspace-id'