The OECD needs to better define what types of income would be targeted by new proposals to revamp how multinationals are taxed, the business community warned in letters to the group.

The Organization for Economic Cooperation and Development is considering four ideas as it seeks a globally agreed-upon solution by 2020 to address some countries’ concerns that multinationals aren’t paying their fair share of tax, or aren’t paying it in the right places. A U.S.-proposed “marketing intangibles” plan is the most promising contender for achieving consensus, a Treasury Department official said in February. That plan would allocate part of a...