The confINSTALL macro defines the program that will be used by make(1) to install sendmail. As distributed, the devtools/OS file for your machine's architecture predefines this value for you. You should not need to redefine it unless you have customized your system in a way that makes that prior definition inappropriate:

define(`confINSTALL', `${BUILDBIN}/install.sh')

Here, we create a definition that tells make(1) to use devtools/bin/install.sh to install sendmail. The expression ${BUILDBIN} is a Makefile macro that defaults to the devtools/bin directory in the source distribution (see confBUILDBIN, confBUILDBIN, for a way to override that default).

Note that this macro also defines how manuals will be installed. It does not, however, control whether to install the manual pages (see the confNO_MAN_INSTALL macro, confNO_MAN_INSTALL). Nor does it define how to install symbolic links (see confLN, confLN).