Pick Encode or Decode. Encoding wraps text in a begin/end block using the XXencode alphabet; decoding reverses it back to the original text.
It packs bytes the same 3-into-4 way but maps 6-bit values onto the alphabet +-0-9A-Za-z instead of raw ASCII offsets, so it survives non-ASCII mail gateways.
Its alphabet avoids characters that older EBCDIC systems and some mailers mangled, making transfers more reliable than uuencode in those environments.
Yes — like uuencode it uses a 'begin <mode> <name>' header and an 'end' trailer, so the format is familiar.