The attribute expression that defines features to be included in the query. The where clause should follow standard SQL syntax similar to that discussed in the document https://pro.arcgis.com/en/pro-app/latest/help/mapping/navigation/sql-reference-for-elements-used-in-query-expressions.htm#GUID-940733A1-D6D1-4200-B290-24CA2E1056D4

If the where clause includes dates, they must be correctly formatted based on the geodatabase datasource used in the service. Refer to Dates and time[https://pro.arcgis.com/en/pro-app/latest/help/mapping/navigation/sql-reference-for-elements-used-in-query-expressions.htm#GUID-85C03D85-F4A5-48FC-8E8C-3F79919430DB] for date formats expected by different data sources.

ArcGISFeatureTable objects expect that any FieldType.GlobalId or FieldType.Guid parameters in the where clause are formatted as: '{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}'. All characters are uppercase, wrapped in curly braces and surrounded by single quotes.

If the where clause is used to populate an OgcFeatureCollectionTable, you can use a CQL2-TEXT or CQL2-JSON string as defined in the document https://docs.opengeospatial.org/DRAFTS/19-079r1.html.