What's the difference between MXAppExitMetric and MXCrashDiagnostic

I got reporter from user with abnormal app process end, metric info is below:

"applicationExitMetrics":{
    "backgroundExitData":{}
    "foregroundExitData":{
    "cumulativeMemoryResourceLimitExitCount":1
}

This seem to be a crash, why not delivered by MXCrashDiagnostic?

MXAppExitMetric An object representing metrics about the types of foreground and background app exits.

MXCrashDiagnostic An object representing a diagnostic report for an app crash.