Keep in mind that CAPI is not compliant with all the hardware products as well as all the applications. So, before using it, check out the list with the compliant hardware and applications supporting CAPI. The list can be found on web address http://www.capi.org/pages/compliant.php
More information about the CAPI interface can be obtained from the official web site on http://www.capi.org/
Chan_capi is a module. Its purpose is to provide the connection between software based PBXs such as Asterisk and an ISDN hardware equipment which supports the CAPI interface.
Chan_capi cannot be used with all ISDN cards. You need an ISDN card which supports CAPI. The list with chan_capi supported hardware can be viewed at the following web address - http://www.melware.org/ChanCapiHardware
There is one weak moment - the online documentation is incomplete, actually it is so small that it is almost missing. However the README file which is coming with the chan_capi package is clear enough and you should not have any problems. The only thing listed as documentation on the chan_capi web site is an error report explaining that although the message says error it is not an error.