What is the phenotype represented by squares with at least one R and at least one F?

RrFf would be the genotype since you have least one of each of these dominant alleles they would have to be included. Phenotype would be whatever these dominant alleles represent. So say that R and F are alleles for brown eye color and black hair. Since you have at least one of each they would both show up and you'd have a phenotype of some brown eyed, black haired individual.