Almost all of the WFS/WFS-T servers, the feature identifiers -represented as gml:id in the XML document- that I have come across were in this format: [LayerName].[IntegerFeatureID], e.g., BarcelonaBuildings.1453. However, sometimes, it can also be like this: 12400.344.561.1. I have looked at the OGC documentation but it doesn't say anything about the structure of the feature identifiers. Should it be integer? Could it be a string? Could it have alphanumeric characters? Should it contain the layer name as part of it?
Is there a specific standard for WFS feature identifiers or can it be anything depending on the server as long as it is unique?