Since the primary help from GNU split says:
Usage: /usr/gnu/bin/split [OPTION]... [INPUT [PREFIX]]
Output fixed-size pieces of INPUT to PREFIXaa, PREFIXab, ...; default
size is 1000 lines, and default PREFIX is 'x'. With no INPUT, or when INPUT
is -, read standard input.
Mandatory arguments to long options are mandatory for short options too.
-a, --suffix-length=N generate suffixes of length N (default 2)
--additional-suffix=SUFFIX append an additional SUFFIX to file names.
-b, --bytes=SIZE put SIZE bytes per output file
-C, --line-bytes=SIZE put at most SIZE bytes of lines per output file
-d, --numeric-suffixes[=FROM] use numeric suffixes instead of alphabetic.
FROM changes the start value (default 0).
-e, --elide-empty-files do not generate empty output files with '-n'
--filter=COMMAND write to shell COMMAND; file name is $FILE
-l, --lines=NUMBER put NUMBER lines per output file
-n, --number=CHUNKS generate CHUNKS output files. See below
-u, --unbuffered immediately copy input to output with '-n r/...'
--verbose print a diagnostic just before each
output file is opened
--help display this help and exit
--version output version information and exit
It looks to me like you need to reorganize your options a bit:
split -a 4 -d -l 50000 domains.xml domains_