Every letter is used to indicate that the access rights the current FTP user has over the following specific actions are granted.
The available permissions are the following listed below:
Read permissions:
"e" = change directory (CWD, CDUP commands)
"l" = list files (LIST, NLST, STAT, MLSD, MLST, SIZE commands)
"r" = retrieve file from the server (RETR command)
Write permissions:
"a" = append data to an existing file (APPE command)
"d" = delete file or directory (DELE, RMD commands)
"f" = rename file or directory (RNFR, RNTO commands)
"m" = create directory (MKD command)
"w" = store a file to the server (STOR, STOU commands)
"M" = change file mode / permission (SITE CHMOD command)
"T" = change file modification time (SITE MFMT command)
Source: https://pyftpdlib.readthedocs.io/en/latest/api.html#users