Variation Selector-10

Alone, it means nothing; it can request a glyph variant.

VS10 is a combining control, not a letter, punctuation mark, or space. Its visible effect depends on the character immediately before it and a defined sequence.

Unicode's wider selector system gives plain text a way to preserve glyph choices that cannot be settled by context or automatic shaping. It covers distinctions in math and punctuation and variants for Myanmar, Phags-pa, Manichaean, and Egyptian scripts. Han ideographs used in Chinese, Japanese, and Korean, plus Mongolian letters, have their own selector systems.

Why does U+FE09 do nothing?

Because FE09 is not a universal style command.

Unicode defines effects for particular base-plus-selector sequences, not for a selector attached to any character. Unicode 18.0's standardized and emoji variation-sequence lists contain no FE09 sequence.

When a sequence is unassigned or unsupported by the software, the base normally appears as usual and the invisible selector is hidden. Emoji text and emoji presentation use VS15 and VS16 for eligible emoji; Han ideographic variants use a supplementary selector range, and Mongolian has separate free variation selectors.

How do I type VS10?

Use Unicode input, then put VS10 after its base.

VS10 has no ordinary keyboard key because it is invisible. In a compatible editor, enter U+FE09 through Unicode code-point input; in source code, use that language's Unicode escape syntax. For example, Python uses \ufe09.