This question beautifully summarises the structure of the sefer we call Mishna Berura. It says that the Chofetz Chaim,
“wrote the sections to his commentary to the Code of Jewish Law: Mishna Berura - concise and easy to learn additional laws and customs to the Shulhan Aruch as well as explanations usually bringing earlier sages such as: Elya Raba, Magen Avraham, and Baer Hetev.
Biur Halacha - a little more in depth than the Mishna Berura where he usually sources his opinions and says why he holds like does.
Shaar HaTziyun - the sources for his words.”
(That question asked, “do the Ashkenazim that follow the M"B follow the words of the Biur Halacha as well?”).
My question is similar but different and in two parts.
It comes about from seeing the Shaar HaTziyun 47 in Simon 253 (see picture below).