U+1D400 The Unicode codepoint the style's characters come from. Two styles can look almost identical and still be different characters with different device support. The codepoint is the only reliable way to tell which one you copied.
IGTTRB Platform support. Green renders reliably. Amber works, but breaks on some devices or in some fields. Struck through is filtered out or rejected by that platform.
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The same style inside each frame. Type above and these update with it, then tap to copy the whole thing. The number is the character count, which matters if you are pasting into a field with a limit.
Italic mathematical letters carry a specific meaning in their original context: in a formula, an italic letter denotes a variable, while an upright one denotes a constant or an operator. That convention is why the set exists at U+1D434 and why it was among the earliest added. Used decoratively it reads as lighter and more editorial than bold, which is usually why people reach for it.
One position in the run tells you something about how Unicode grew. Lowercase italic h is reserved, because U+210E had already been assigned to the Planck constant, the physics symbol โ. Rather than duplicate a character that already existed and looked identical, the standard left the maths slot empty and pointed at the physics one. So the h in your italic text is, formally, a physical constant. It looks exactly right and behaves exactly like a letter; it just has an unusual name.
Everything else in both cases is contiguous. Digits are not covered, so numbers stay plain.
Support is essentially as good as bold. These characters have been in the standard since 2001 and are covered by the fonts shipped with every current operating system. Where italic is weaker is at small sizes: the slant reduces the effective stroke contrast on a low-resolution display, so it holds up less well than upright styles in a phone bio. Bold italic exists at U+1D468 and is fully contiguous, with no reserved positions at all.
Secondary lines and short asides.
Slanted text has meant 'aside' in print for centuries, and screens inherited the convention, so an italic line beneath a heavier one reads as supporting information without needing to say so.
On its own it is subtle. At bio size the slant reduces apparent stroke weight, so a lone italic line can read as unstyled to anyone not looking for it.
Bold italic solves that at the cost of some restraint, and is fully contiguous with no borrowed characters, which plain italic cannot claim.
Because U+210E, the Planck constant, was already in Unicode and looked identical to an italic h. Rather than encode the same shape twice, the standard reserved the maths position and reuses the existing character. It renders and behaves like a letter. The name is just a historical artefact.
Bold italic is fully contiguous with no reserved positions and holds definition better at small sizes. Plain italic is lighter and reads as more restrained, which suits some contexts, but it loses more clarity on a phone screen.