The Bus has two ways the pilot can manually control lateral and vertical modes.
1 - Heading / Vertical Speed
This is the default mode. When you pull the heading knob you can steer the aircraft by heading. When you pull the VS knob you get.... vertical speed.
So, for example, approaching LGW under vectors, where you are required to make a constant descent with no level-offs, you might make a vertical speed selection to avoid that happening. The reason for this is that when you are in 'heading', the aircraft will not manage the descent profile for you; you will go into 'open descent' mode, the engines will be idled and you may be coming down too fast to avoid a level off.
2 - Track / Flight Path Angle (FPA)
Pushing the button next to the heading knob switches to TRK/FPA. Now when you select a course, the aircraft flies exactly that.. the track you asked for. Or, in other words, a wind-corrected heading.
The selected vertical mode now becomes FPA and can be selected in 0.1 degree increments.
Very useful for flying a VOR approach - lock onto the final approach track in Track mode, and just before the top of descent mode, select -3.0 degrees (or whatever is appropriate for your procedure) and you will commence a continuous descent at that angle, without having to step-down, as done by light aircraft.
You cannot mix modes.. i.e., you cannot have Heading and FPA, or Track and V/S.
There are many ways to skin a cat - LOC only approach for example - some guys prefer V/S, some prefer FPA for the vertical mode.
Push the heading knob for NAV mode. Pull it for Heading/VS or Track/FPA.
Push the alt knob for managed climbs / descents and cruises - the profile will be controlled by what is in the FMGS and the engines will spool up/down as required to maintain your profile and speed.
Pull it to go into open climb (max climb thrust, v/s controlled by speed) or open descent (idle, v/s controlled by speed).
The real aircract goes into NAV mode automatically, fairly shortly after departure to follow the programmed SID.