This character is a Other Symbol and is commonly used, that is, in no specific script.
The glyph is a square composition of the glyphs Latin Capital Letter M, Latin Capital Letter B. Its East Asian Width is wide. In bidirectional text it is written from left to right. When changing direction it is not mirrored. U+3386 offers a line break opportunity at its position, except in some numeric contexts.
The Wikipedia has the following information about this codepoint:
The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix mega is a multiplier of 1000000 (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes of information. This definition has been incorporated into the International System of Quantities.
In the computer and information technology fields, other definitions have been used that arose for historical reasons of convenience. A common usage has been to designate one megabyte as 1048576bytes (220 B), a quantity that conveniently expresses the binary architecture of digital computer memory. Standards bodies have deprecated this binary usage of the mega- prefix in favor of a new set of binary prefixes, by means of which the quantity 220 B is named mebibyte (symbol MiB).