Relays can have a number of different levels of sealing, from being completely open to the atmosphere, to full hermetic sealing. Which you need depends on your application, but also how you plan to solder it to the board.
The technical guide would seem to be your go-to source. Take a look at page 2 (by the document's page numbering), and see how they define their levels of sealing. I would assume "Flux Protection" maps to "Semi-Sealed", and "Sealed" maps to "Fully Sealed", but you may need to contact Omron if you need more assurance than that.
If I'm right, then the major difference is often that during PCB Assembly you can do automatic washing after reflow with sealed relays. But read through the technical data to be sure that you're making the correct choice for your application. Other factors like environment and conformal coating can play a part.