Hi again sarahr202
Regarding your question (Are data plane logs considered as SYSLOG?), the answer is yes.
Note that SYSLOG is just a standard for message logging so either controlplane or dataplane log messages are sent using this standard and format. Note that SYSLOG allows for the separation of the software that generates messages (Junos in this case), the system that stores them (your syslog server), and the software that reports and analyzes them (any reporting/management tool, if present, that uses this syslog information to present statistics or alarms in your network).
Being said this, it is still important to understand the difference between controlplane logs and dataplane logs.
Controlplane messages are related to events on your box (a user that just login to the device or a high temperature alarm) and are configured under the [edit system syslog] hierarchy:
https://kb.juniper.net/KB16502
Note that these logs can be stored locally in the SRX or sent to an external host as explained in the above KB article.
Dataplane messages, known as security-logs or traffic-logs, are messages related to the traffic that is being forwarded by your SRX. These logs are related to sessions and are configured under [edit security log] hierarchy:
https://kb.juniper.net/KB16509
Note that these messages can be sent to an external host (highly suggested) by using the "stream" mode, and they will be sent directly from the dataplane of the device hence not affecting your Routing-Engine.
Also they can be stored locally in the SRX, using the "event" mode but this will make the SRX to send the logs from the dataplane to the controlplane and depending on the rate of the logging this can affect your Routing-Engine (which is the component in charge of the controlplane of any junos device).
I hope this info is helpful.