threads = ['thread1','thread2']
def branches = [:]
for(int i = 0; i < threads.size(); i++) {
def thread = threads[i]
    branches[thread] = {
        println 'labeled'
        node('master') {println "labeled"}
        println 'not labeled'
        node('master') {println "also not labeled"}
    }
}
parallel branches

Running: Execute sub-workflows in parallel : Start
[thread1] Running: Parallel branch: thread1
[thread2] Running: Parallel branch: thread2
[thread1] Running: Print Message
[thread1] labeled
[thread1] Running: Allocate node : Start
[thread1] Running on master in /var/lib/jenkins/workspace/test@3
[thread2] Running: Print Message
[thread2] labeled
[thread2] Running: Allocate node : Start
[thread2] Running on master in /var/lib/jenkins/workspace/test@4
[thread1] Running: Allocate node : Body : Start
[thread2] Running: Allocate node : Body : Start
[thread1] Running: Print Message
[thread1] labeled
Running: Allocate node : Body : End
[thread2] Running: Print Message
[thread2] labeled
Running: Allocate node : Body : End
Running: Allocate node : End
Running: Allocate node : End
Running: Print Message
not labeled
Running: Allocate node : Start
Running on master in /var/lib/jenkins/workspace/test@3
Running: Print Message
not labeled
Running: Allocate node : Start
Running on master in /var/lib/jenkins/workspace/test@4
Running: Allocate node : Body : Start
Running: Allocate node : Body : Start
Running: Print Message
also not labeled
Running: Allocate node : Body : End
Running: Print Message
also not labeled
Running: Allocate node : Body : End
Running: Allocate node : End
Running: Allocate node : End
Running: Execute sub-workflows in parallel : Body : End
Running: Execute sub-workflows in parallel : Body : End
Running: Execute sub-workflows in parallel : End
Running: End of Workflow
Finished: SUCCESS