Hi, K1mffrey
It is important to understand the two major type of logs that can be saved by the SRX: data-plane logs vs control-plane logs
Control-plane messages are related to events on your box (a user that just loged in 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.
Data-plane messages, also 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 data-plane to the control-plane and depending on the rate of the logging this can affect your Routing-Engine (which is the component in charge of the control-plane of any junos device). Any of the mentioned modes are configured under [edit security log] hierarchy.