The Boolean setting of $FCTraditionalFormOutput determines which output format type should be used in the notebook front end when FeynCalc is loaded. If set to True, FeynCalc will activate the TraditionalForm output. Otherwise, the StandardForm output (Mathematica’s default) will be used.
This setting only changes the output format of the current notebook, i.e. it is not persistent and will not modify the global options of Mathematica.
If unsure, it is recommended to set $FCTraditionalFormOutput to True, so that you can benefit from the nice FeynCalc typesetting for various QFT quantities.
Overview, FCEnableTraditionalFormOutput, FCDisableTraditionalFormOutput.