Hello, i was reading the manual, and it is stated there that box prompts are a required data to be inputed and dash prompts are optional.

 

I went to the RTE page, where origin airport is in dash prompts and destination is in box prompts, tried to set only destination airport, but one can't do it, you must set the origin first.

 

It is quite logical, that the fmc needs both points for its route managment, but why then use different prompts ?

I think it's because the dashed prompts data can come from other pages in the FMC, but the box prompts actually come from your head, and are not available until you "tell" the FMC. That's my interpretation anyway.