The control interface is used widely for many switches, sliders, etc. which are accessed from the user-space. Its most important use is the mixer interface. In other words, on ALSA 0.9.x, all the mixer stuff is implemented on the control kernel API (while there was an independent mixer kernel API on 0.5.x).
ALSA has a well-defined AC97 control module. If your chip supports only the AC97 and nothing else, you can skip this section.
The control API is defined in <sound/control.h>. Include this file if you add your own controls.