It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionized—HII is H+ in other sciences—III for doubly-ionized, e.g. OIII is O++, etc.
This line has a very low transition probability, so it requires large amounts of hydrogen gas for it to be seen.
[3] Mapping HI emissions with a radio telescope is a technique used for determining the structure of spiral galaxies.
HI regions effectively absorb photons that are energetic enough to ionize hydrogen, which requires an energy of 13.6 electron volts.
They are ubiquitous in the Milky Way galaxy, and the Lockman Hole is one of the few "windows" for clear observations of distant objects at extreme ultraviolet and soft x-ray wavelengths.