In a scan or merge operation, clrmamepro detects which archives you're
scanning/merging and uses this type for all operations. The Rebuilder
lets you select which format the destination archive should get.

While rar/7z read support is included since ages internally, the
packer-write support is realized via external tools (7z.exe and
winrar.exe) due to license reasons (rar) and pure lazyness (7z) ;).

The definitions are stored in 'packer.ini' where you specify the executable
and parameters for compression and deletion of files.

Let's have a look at the default packer.ini entries:

7zip_compress_exe  = 7z.exe
7zip_compress_para = a -y -t7z %1 %2
7zip_delete_para = d %1 %2

rar_compress_exe  = winrar.exe
rar_compress_para = a -y %1 %2
rar_delete_para = d %1 %2

First line tells cmpro the exe name of the 7z or rar commandline packer.
When no full path is given, be sure it's included in your %PATH% environment.
Second and third line is the commandline parameters for adding and removing
a file. %1 and %2 are placeholders for the archive name and the file name
which gets deleted or added from the archive. clrmamepro needs these, they get
replaced during runtime. For the other parameters, check out 7z and/or
winrar's documentation.

I recommend not to change packer.ini unless you're familar with the cmdline
parameters of winrar/7z.