Documentation says:

Named sessions

Named sessions represent an additional level of plugin parameters and can be used to define separate sets of authentication parameters for each of the instances being monitored. Each named session parameter should have the following structure:

Plugins.<PluginName>.<SessionName>.<Parameter>=<Value>

It seems that according to examples that follow and the rest of documentation (e.g. see here, here, etc.) the pattern should be:

Plugins.<PluginName>.Sessions.<SessionName>.<Parameter>=<Value>