What ⊂ means
Sometimes; the author's convention decides. Under the strict convention, every element of A is in B, and B has an element outside A. Under the inclusive convention, A and B may be equal. ⊊ explicitly excludes equality.
In 1890, Ernst Schröder used ⊂ for “subordination,” applying the idea of containment to sets and logic. In set notation, ⊆ allows equality; ⊊ excludes it explicitly.
Formal mathematical notation; in casual use, it signals a mathematical context.
When equality matters, follow the convention stated in the text.
Don't confuse it with
3Questions people ask
1Why does ⊂ show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type ⊂
⊂\2282Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.