eSet (package 'Biobase').myCounts derives from eSet, we have used the slot assayData to store all the expression data, phenoData to store the factors with the conditions, featureData which will contain the variables Length, GCcontent, Biotype, Chromosome, Start Position, End Position for each feature. It has been used the slot experimentData derived from MIAME-class which will contain the type of replicates (biological replicates, technical replicates or no replicates at all).If you need further information to know the methods that can be used, see eSet, AnnotatedDataFrame-class, MIAME-class.