An ArrayBufferView is typically a "window" onto an ArrayBuffer |buffer| starting at |byteOffset| for |byteLength| bytes; the buffer may start before the offset and continue past the length.

decode() step 3 says:

If input is given, append the bytes from input's buffer, starting at input's byteOffset, to the stream.

Since this is going into detail about input's |buffer| and |byteOffset| it should probably mention |byteLength| as well, e.g.:

If input is given, then given input's buffer, byteOffset and byteLength, append byteLength bytes from buffer starting at byteOffset to the stream.