What qualifies as a genuine reason for a multiple entry Non-O e-visa in Thailand?

soon, i will apply for a Non-O e-visa (retirement) . On the e-visa site, they give you an option to chose between One Entry or Multiple Entries, but they warn you that, if you ask for Mulltiple, you "prove that you have a genuine reason to visit Thailand on a regular basis" and there will be a fee difference also. What i was thinking of doing was, while in Thailand (for about 4 months), maybe to visit one, maximum 2 neighbour countries. Would that be a "genuine reason"? What prove should i give them in that case? Am i right to say that, if my visa is only One Entry, i can not visit any other country while staying under it's validity?