An Expert Witness is used to record the details and the qualifications as to what makes them an expert in a specific area.
For example an assessor can assess a learner on using Excel but may not be an expert in this function. The assessor would then gather all evidence but will require a witness testimony as supporting evidence from an expert user of Excel. This could also be used for specific machinery, software, etc.
Guidance on assigning an Expert Witness to an assessment can be found here.