Halliday & Matthiessen (2014: 276):
Within the other major domain of attribution, the ‘material’ domain, we find an analogous situation where Attribute denotes a material quality equivalent to the Process of a ‘material’ clause, and may be formed as a participle from a material process verb; for example,
they said that our disk was corrupted when it arrived …
In addition, while ‘material’ clauses are typically construed with the present-in-present, ‘relational’ ones are construed with the simple present. Thus [sic] has to be interpreted as ‘material’ – market research is being conducted this week; and our disk was being corrupted (when it arrived) would be ‘material’ rather than ‘relational’.