User`s guide

If the command field is preceded by a drive specification id:), the CCP attempts
to open the command or submit file on the specified drive. Otherwise, the
CCP attempts to open the file on the drives specified in the drive chain.
The drive chain specifies up to four drives that are to be referenced in sequence
for CCP open operations of command and submit files. If an open operation
is unsuccessful on a drive in the drive chain because the file cannot be found,
the CCP repeats the open operation on the next drive in the chain. This
sequence of open operations is repeated until the file is found, or the drive
chain is exhausted. The drive chain contains the current default drive as its
only drive unless the user modifies the drive chain with the CP/M 3 SETDEF
utility.
When the current user number is non-zero, all open requests that fail because
the file cannot be found, attempt to locate the command file under user zero.
If the file exists under user zero with the system attribute set, the file is
opened from user zero. This search for a file under user zero is made by the
BDOS Open File function. Thus, the user zero open attempt is made before
advancing to the next drive in the search chain.
When automatic submit is in effect, the CCP attempts to open with the first
filetype, SUB or COM, on all drives in the search chain before trying the
second filetype.
In the banked system, if a password specified in the command field does not
match the password of a file on a disk protected in Read mode, the CCP file
open operation is terminated with a password error.
If the CCP does not find the command or submit file, it echoes the command
line followed by a question mark to the console. If it finds a command file
with a filetype of COM or PRL, the CCP proceeds to step 4. If it finds a
submit file, it reconstructs the command line as described above, and repeats
step 3 for the command, SUBMIT.COM.
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1.6 System Operation CP/M 3 Programmer's Guide