The Free On-line Dictionary of Computing (30 December 2018):

uuencode

    (Unix-to-Unix encode) A Unix program for
   encoding binary data as ASCII.  Uuencode was originally
   used with uucp to transfer binary files over serial lines
   which did not preserve the top bit of characters, but is now
   used for sending binary files by e-mail and posting to
   Usenet newsgroups etc.  The program uudecode reverses the
   effect of uuencode, recreating the original binary file
   exactly.

   Uuencoded data starts with a line of the form

   	begin  

   where  is the files read/write/execute permissions as
   three octal digits and  is the name to be used when
   recreating the binary data.

   Uuencode repeatedly takes in a group of three bytes, adding
   trailing zeros if there are less than three bytes left.  These
   24 bits are split into four groups of six which are treated as
   numbers between 0 and 63.  Decimal 32 is added to each number
   and they are output as ASCII characters from 32 (space) to
   32+63 = 95 (underscore).  Each group of sixty output
   characters (corresponding to 45 input bytes) is output as a
   separate line preceded by an 'M' (ASCII code 77 = 32+45).  At
   the end of the input, if there are N output characters left
   after the last group of sixty and N>0 then they will be
   preceded by the character whose code is 32+N.  Finally, a line
   containing just a single space is output, followed by one
   containing just "end".

   Sometimes each data line has an extra dummy character added to
   avoid problems which mailers that strip trailing spaces.
   These characters are ignored by uudecode.

   Despite using this limited range of characters, there are
   still some problems encountered when uuencoded data passes
   through certain old computers.  The worst offenders are
   computers using non-ASCII character sets such as EBCDIC.

   Base 64 encoding is probably now more commonly used than
   uuencode.

   (2004-07-17)