The percentage held or owned by an individual or institution is dependent of the percentage of the number of shares they own out of the total number of outstanding shares. For example, if a company has 10 million shares and a person owns 10% of the company, then they own 10% of 10 million shares, which is 1 million shares.
In any company, the shares are owned by either a member of the general public, or a member of the company, or the company itself.
Held Percentage Insiders
HELD_PERCENT_INSIDERS is the percentage of all shares owned by members of the company (e.g. employees). For example, in AAPL, the percentage owned by insiders is 0.0066% currently.
Held Percentage Institutions
HELD_PERCENT_INSTITUTIONS is the percentage of all shares owned by institutions (e.g. the company itself, a hedge fund). For example, in AAPL, about 0.58% of the company is owned by institutions currently.