Holy of Holies in the Heavenly Sanctuary is the term that Seventh Day Adventists use to refer to the exact place where Christ entered to perform his judgement. Therefore, you could assume that this is the place where, according to this faith, Christ actually resides in heaven.
This concept became accepted by the congregation in 1844 when, according to the history, Adventists expected the second coming of Christ to occur. As the event (called the "Great Disappointment") did not take place, a subgroup within the congregation declared that what Christ actually did was enter the Holy of Holies in the Heavenly Sanctuary.
In Seventh Day Adventist theology, the term "sanctuary" does not really refer to the physical part of a church on Earth. Instead, SDA doctrine holds that Jesus Christ is currently in the holiest sanctuary in Heaven. He has been there since 1844.
The idea here is that there is a sanctuary in Heaven that God created. It is sort of the heavenly analogue to the holiest places in the Jewish temple in the Old Testament. Since 1844, Jesus has been in the sanctuary in Heaven. He has been conducting the final phase of his ministry -- his mission to help us people. Once he has finished with this work, he will come to Earth again.