If you try to AutoApply a retention label using the Office 365 Security & Compliance Center you might get an error message if you use KQL (Keyword Query Language):
This is because a KQL query for AutoApply only allows you to use certain "Queryable" Managed Properties. Some of them are listed here: https://docs.microsoft.com/en-us/microsoft-365/compliance/keyword-queries-and-search-conditions#searchable-site-properties
But the Managed Properties in the article listed above are not the only ones. Although you cannot use custom Manged Properties (that are queryable), you can use the predefined ones like RefinableString00 etc. Make sure that you don't use the alias name in the query. This is valid KQL but doesn't work in the AutoApply policy.
You can associate crawled properties to managed properties in the SharePoint Admin Center | More Features | Search | Manage Search Schema.
My thoughts and findings about SharePoint 2016/2013/2010 and SQL Server