Difference between allele1 vs. allele2 in TCGA MAF files?

Hi all,

TCGA documentation for the MAFs (mutation annotation files) have Allele1 and Allele2 versions of similar values; what is the difference between these alleles?

For instance, I am interested in Tumor_Seq_Allele but not sure whether to use Tumor_Seq_Allele1 or Tumor_Seq_Allele2. I am looking at data for ACC (Adrenocortical carcinoma) tumors, and for many of the mutations, Tumor_Seq_Allele1 is actually the same as the reference allele. Which one is the actual mutation?

Thank you in advance! For reference, this is the documentation I am looking at: docs.gdc.cancer.gov/Data/File_Formats/MAF_Format/