Clear is a procedure used to ensure that characters left over from prior calls to Decode are removed before starting a new decoding operation. 

TIdDecoderMIMELineByLine maintains an internal buffer for decoding operations that use Base64-encoded values that are not a multiple of 4-bytes. In other words, the FillChar may not have been appended to the end of the encoded values. 

Use Clear to ensure that these remaining values are discarded proper to calling Decode for a new set of Base64-encoded values.

Decode 

FinishDecoding