Why are there two options for booking first class semi-flexible tickets?

I'm not sure which airline you're referring to, but I know when I bought my United flights, they had a not-entirely-refundable option and a pricier fully refundable option.

Sorry. This is for train tickets!

Got it! I won't be able to help with that, but for others who may be able to, which country are you buying train tickets for? And from which website?

For trains in which country? What website are you using?
(Oops, posted at same time as CWsocial.)

I am booking a ticket from Kaiserslautern, Germany to Prague.
All of the tickets I have looked up have two options for semi-flexible first class tickets with the same descriptions, but difference prices.

Make sure that you are using the German Railway Website
https://www.bahn.com/en

I looked up your route and saw that there were two types of flexible tickets. One included seat reservations and a few other features. If you use the website above, it will clearly tell you what is included in each fare and what the differences are.

Are you perhaps seeing the 2 pricing columns for "Savings Fares" and "Flexpreis" ?

From their descriptions:
If you have a saver fare ticket, you can only travel on the service indicated on the ticket.

With Deutsche Bahn's flexible ticket you can simply hop on the train and leave. Travel whenever you want on whatever train suits you. You can even interrupt your journey.

I was using Eurail's website via Rick Steves.
One of the legs of the trip is long, so I want first class with seat reservations.

Definitely use the German Railroad site and it will be quite easy to see what you want. Have fun!

You'll almost certainly save money and may have more choices of departure time if you buy tickets directly from the rail company. Most third-party sellers mark up the tickets and add a service charge.

You should double check fares at the E-Shop of Czech Rail. They often (but not necessarily) have better offers than the DB. E.g., fares for Kaiserslautern - Prague a week out start at czk 1265 ~ 52€ for train specific ("unflexible") tickets.

You can also seek guidance from the Man in Seat 61

I found that the website that sla019 recommends was the best option when I took the train from Prague to Berlin and back in 2018. It was easy to navigate and get exactly what I needed. I settled on this website after researching many options.