In my edition of the Aruch Hashulchan, there is often a paragraph that has Rashi script written inside brackets (see pic). What is the Rashi script? Is it the mechaber or someone/something else? I couldn't find any info on this in the opening pages of the Oz VeHadar edition of the Aruch Hashulchan.
This exists in all editions of the Aruch HaShulchan. See for example the first page of this older edition.
Perhaps you couldn't find anything on this because unexpectedly the introduction to the work is in Choshen Mishpat. This is because it was the first section of Shulchan Aruch that he wrote.
There, note 8, he explains that he put these pieces in square brackets (and a different font), because they're not specifically to tell you the final halacha. Rather, they're his novella in understanding of the gemarra or rishonim. To not get bogged down with these novella he put them in brackets so you could skip them.