The 'Trust level' field is for if your school is part of a Multi Academy Trust (MAT) and you need to add staff members to your schools Scholarpack that work for the Trust rather than the school directly.
This does not do anything other than highlight that that particular staff member works for the Trust.
This is recorded via the staff members profile. Click the white cog next to their name.
There's a field called 'Trust Level' that can be set to either Yes or No