Florescent lighting has characteristics that affect color rendition. This color shift is not usually visual to the human eye but will cause adverse effects on film.
When shooting under "daylight" florescents your pictures will have a purplish tone. Hoya's FL-D filter corrects and restores a normal color balance.
In industrial situations you might encounter "warm white" florescents which will give a greenish tone, requiring Hoya's FL-W corrective filter.
Both filters are available in coated and multi coated (stronger flare prevention) versions and all popular sizes.
One reason why the Hoya 67 FL-W is popular is because of its Clear optics.