username,
Final answer is:
“em0 and em1 are internal communications to the RE as stated in KB21706.
Because the control-link ports are used for communication between the REs in a cluster, in SRX1500 and SRX4K they are linked to em0 which is the internal link used for communication of REs for cluster purposes. However the physical status of the control-link ports don’t necessarily relate to the status of em0 link and vice versa.
Interface em1.32768 is a subunit of em1 that is used for internal communications between the RE and other components, for instance for communications with the linux host OS on which the Junos runs as a vm.”
I hope the above information answer your question.