Mathematical Sans-Serif Capital E

It is a separate mathematical E; the formula supplies its meaning.

Mathematical letter styles can distinguish symbols even when they share a basic letter. 𝖤 marks E in sans-serif math style; the formula or its author tells you what that symbol stands for.

That distinction motivated a 1999 proposal from the STIX mathematical publishing project: separate styled characters let plain text preserve differences already used in formulas. It included sans-serif as one of the mathematical alphabets, without assigning this E a fixed value.

Can I type 𝖤 as a formula style?

Yes: request sans-serif styling in your math notation.

In LaTeX math mode, \mathsf{E} requests a sans-serif E. In MathML, put E in an <mi> element with mathvariant="sans-serif". The math renderer uses its chosen font to draw the letter.

Compatibility normalization such as NFKC folds 𝖤 to ordinary E, removing the style distinction. Preserve the mathematical character or use style markup when that distinction matters to the formula.