This configuration setting controls the CPU architecture of the process used to build the project.
Because MSBuild itself will always inherit the CPU architecture of the hosting process, this setting controls the platform that will be used to execute all MSBuild tasks during the build.
This setting will not control the CPU architecture of an assembly built from the project. To control this, instead refer to the Use Build Platform configuration setting.
Visual Studio usually defaults to launching MSBuild within an x86 process, so switching this setting to x64 is considered unconventional. MSBuild itself can be launched from the command-line as either x86 or x64, depending upon the environment. Because it is not always possible to control the CPU architecture of the build process outside of NCrunch, it is recommended that you do not engineer your projects to be dependent on a specific CPU architecture.
Some custom MSBuild tasks are designed to only work with a specific CPU architecture (for example, some Visual Studio SDK tools will only run under x86).
If you are experiencing problems with the NCrunch build process running out of available memory during build or instrumentation, adjusting this setting to x64 can allow the process to work past the usual 2GB memory limit for x86 processes.