Create a new submission type called "Poster" from under Submission Types.
Then go to the Proposal submission form and create fields to display only for this particular submission type (create a field and select "Poster" submission type for that field in the field settings; see this screen capture: https://gyazo.com/938351b6428846a258c6e35714e97275 ).
This will allow authors to select the poster submission type when submitting a poster and that would then load the fields related to the poster submission type.
