In Adventist theology, person and being are used synonymously with no real distinction made. They define both as having a “literal, tangible form” consisting of physical matter. God the Father, God the Son and the Holy Spirit all have a tangile, literal form.
In Christian theology, person is being used to refer to the ability to will, self-reflect, be spoken to, show emotion, etc. This is not restricted to only physical beings. For example, spirits and angels possess this ability and they are not physical beings. Being is defined as the substance, essence and mode of existence.
These terms have been utilized by the Christian church to help denote the plurality and singularity of God without sliding into polytheism. The Adventist Church’s rejection of such (as being pagan and Greek philosophy laid over the Bible) has led to them fashioning a god in their own image.
While the Seventh-Day Adventist Church has borrowed the term “trinity,” they do not teach orthodox, Christian trinitarianism.
The Adventist Church is defining these terms this way—ultimately—because of the writings of Ellen G. White, but also because of their physicalist worldview.