Freenet has three different security level settings, each of which can be set to three or four different levels (low, normal, high, and possibly maximum). Which one you should select will depend upon how concerned you are about different classes of attacks.
The three settings are the strangers setting, the friends setting, and the physical setting. These control, respectively, how hard Freenet should try to defend against attacks on your anonymity from strangers on the Internet, from your Friends who you have Darknet connections to, and from Bad Guys physically seizing your computer.
Effects of different settings
Relative to commit
line 341: if networkThreatLevel is changed to maximum, set ignoreLocalVsRemoteBandwidthLiability to true, otherwise set to false (default)
line 154: if opennet is turned on and security level changes to low, disable oneConnectionPerAddress, otherwise leave it on
line 251: if network threat level is changed to low, set paddDataPackets to false, otherwise set to true
line 1466: if networkThreatLevel is high or maximum (opennet disabled), and friendsThreatLevel is high, disable routeAccordingToOurPeersLocation, else it is enabled
line 1482: similar test to above, but triggered by change in friendsThreatLevel
line 1814: if networkThreatLevel is high or maximum, turn off opennet
line 2396: if networkThreatLevel = low || normal, and physlevel is low, write local requests to datastore, otherwise don't.
line 2407: similar to above, but triggered by change in physicalThreatLevel
line 2491: turn off slashdot cache if networkThreatLevel = maximum
line 204: if networkThreatLevel is changed to low, set alwaysLoadOfficialPluginsFromCentralServer to true, else false