The validity codes for data in the Tobii Pro SDK denotes if the data is trustworthy or not. A common use for the validity codes it to filter out invalid data before doing any analysis.
In the Tobii Pro SDK, each individual kind of data relating to the eye is provided with its own validity code which can only have one out of two possible values: valid or invalid. If the value is set to be invalid, it is recommended that the data is discarded from the analysis as the internal algorithms in the eye tracker did deem it to be of low quality for one reason or another. What causes the algorithms to classify it as invalid varies, but could be everything from there not be any information to calculate the data from (poor eye images) to inconsistencies in calculations caused by external or internal noise of some kind.
The benefit of having individual validity codes per data type is that even if the data for one type is poor, the data for the type of interest in your specific study might still be ok. This means that less data needs to be discarded. However, how the validity codes are calculated is somewhat dependent on the algorithms used in each eye tracker. Older models of eye trackers will still calculate just one validity value for all gaze related data which through the Pro SDK will be used for all data types.