How do I decide which funding mechanism to apply for?

Determining the differences between mechanism, such as between an R03 and an R21, can be confusing. NIH has provided a helpful information sheet that breaks down the different available mechanisms with a brief description of each.

An R03, for example, is designed to support a smaller-scale research project, with a smaller budget and time period than an R01. An R21, meanwhile, is designed for completing exploratory research, often to gather data to support a future R01 application.