If you enter a `source:layer:id` combination that cannot be found then the `features` array in the response will continue a different number of elements than the number of requests. This will be most noticeable in multi-id requests since your request may have 3 ids requested but only 2 results returned. The reason for this is that the `features` section of the response is GeoJSON-compliant and there is currently no way to convey an exception condition (not even an empty JSON element, `{}`). For this reason, if your application is dependent upon the results mapping directly to the individual input requests in order, then you'll have to do your own bookkeeping to handle with exception conditions.
## Valid combinations
Some combinations of `sources` and `layers` are valid while others aren't. Please use the following table for reference:
