According to the Saint Louis Chess Club:
- National Events. Co-sponsored by the USCF, these events offer official U.S. titles and most are open to bidding by USCF affiliates. (Meaning that any affiliate - such as a club or organization holding USCF affiliation - may make a bid to host these tournaments.)
- Grand Prix. Major events, usually open to all, listed in date order. See "USCF GRAND PRIX" for more information.
- American Classics. Events that are open to all and have attracted more than 400 players for three consecutive years.
- Heritage Events. Tournaments held for at least 25 years.
I could not find a "USCF GRAND PRIX" section on the Saint Louis Chess Club's website, but Wikipedia says:
The USCF Grand Prix is a set of chess tournaments for prize money
rated by the United States Chess Federation. In general, a tournament
must have at least $300 in guaranteed prizes to award "Grand Prix"
At the end of the year, prizes are awarded to players with the most
points. The first prize is usually $10,000.
These prizes provide incentives to grandmasters to play in small
regional tournaments which they would otherwise avoid.
I could not find a specific definition of "Enhanced Grand Prix", but from context of things I did find, it appears that "Enhanced Grand Prix" refers to tournaments that offer a larger number of Grand Prix points due to having a larger guaranteed prize pool. It also appears (according to this news item) that holding an "Enhanced Grand Prix" event requires the organizer to make a mandatory contribution to the USCF Professional Players Health and Benefit Fund.
"Junior Grand Prix" is a separate Grand Prix competition for Junior players (under 21 years old).