Z signal on rotary encoder?

[Erich Müller <emcuser-at-gonzales.swb.de>]

Hello,


starting to collect components for machine construction. Got an encoder
(rotary 2500 imp/rev) that lists on its label, apart from the usual power
+/- supply leads the ground shield braid the A and B signal leads also a 
Z-lead.

What signal would this Z-lead carry? Can it safely be ignored or do I need
to connect it if I ever get to put this encoder to its intended use? 

Thanks,
Erich