CA1704 - Microsoft seems to be blocking the word 'Multi'?

try adding this to your customdictionary

<Deprecated>
  <Term PreferredAlternate="Multi">multi</Term>
</Deprecated>

You can refer to this feedback ticket. This is apparently by design.

The behavior you are seeing is because Code Analysis ships with a pre-defined custom dictionary that always overrides any other custom dictionaries when there is a conflict. In this case “multi” is listed as an <unrecognized> word in the default FXCop dictionary.

To fix this issue, open the default dictionary %VS Install Directory%\Team Tools\Static Analysis Tools\FxCop\CustomDictionary.xml and comment out or remove the “multi” entry from the <unrecognized> words section; "multi" will no longer be flagged as a spelling error in Code Analysis.