Hi,
the FeynCalc Book is not quite up to date at the moment. Unfortunately, we don't have enough man power to update the documentation simultaneously with the code, see e.g.
https://github.com/FeynCalc/feyncalc/wiki/Documentation
The bar is fine. It just signifies that the given object (e.g. Dirac matrix or a 4-vector) lives in 4 dimensions. There was exactly the same question on the mailing list a couple of months ago
http://www.feyncalc.org/forum/0881.html
so my old answer to that still applies:
http://www.feyncalc.org/forum/0882.html
Cheers,
Vladyslav
