"CPU X : System" isn[SQ]t a standard or universally recognized term. It[SQ]s likely a shortened or customized label used in a specific context, such as:

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A custom system identifier:

Someone might use this to label a particular CPU within a system, perhaps in a log file, a configuration script, or a monitoring tool. "X" could be a placeholder for a specific CPU model number, a unique identifier, or just a general designation.

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Part of a larger system name:

The entire system name might be something like "CPU X: System Alpha" where "System Alpha" is the overall name and "CPU X" is identifying a specific component.

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An abbreviated display:

Some software might display CPU information in a truncated format for brevity.

Without more information about where you encountered this phrase, it[SQ]s impossible to give a precise definition. If you can provide the source (e.g., a screenshot, a log file excerpt, the software that displays it), I might be able to give you a more accurate answer.