It is normally quite rare to have that attribute populated however, discovery will populate that attribute in 4 scenarios:
1. Inventory discovery. The presence of UD Agent isn’t relevant for this situation . Basically, if the scanner can retrieve the chassisType value then it will be added in the scan file
2. SNMP for several Cisco devices
3. SNMP for Cabletron devices
4. Normalization rules which based on some input rules can have the output rule to populate this attribute with a predefined value.
Higher incidences of population of this attribute may be obtained if SNMP discovery is used for the supported devices referred to in points 2) and 3) above.
Alternatively - if in a discovery scenario where population of that attribute is expected and the results come back with it unpopulated, investigation would need to be done, perhaps by running a scanner on debug on a working host and a failing host to see how the value is retrieved/ not retrieved ? One reason for a non-discovery could be that it’s a VM and there’s no chassisType (as an example) Or it could be down to a rare OS and the value is located in a place that's different from where the scanner is expecting to find it (as another example)