program &>> result.txt
is equivalent to
program >> result.txt 2>&1
Redirect and append both stdout and stderr to file result.txt.
& means both standard output (1>) and standard error(2>).
>> means append to end of the file.
You can use 1>>a 2>&1 instead of &>>
Eg. date test >>file 2>&1
Read Me:
https://tldp.org/LDP/abs/html/io-redirection.html