Sample measurements is generally considered to be the raw data for a trial. It links measurement values and meta data to Trial Units with their corresponding Trait, instance and sampling level (Sample Type).
The Sample Type system provides the same generic levelling functionality that Unit Position Factors offer but at a sub design level. Common uses for sample type factors include plant, tiller, leaf and depth measurements. It may be used for subset grouping of measurements.
Trait Instance refers to repeated measurements on the same Trait, within the same Trial Unit at the same sampling level. The repeated measurement might be on different days for traits such as zadoks growth scale, or on the same day for multiple readings of height taken on the same Trial Unit.