A Fact in Object-Role Modeling is an instantiation of the values that can fill the Roles of a Fact Type.
For instance, in a warehouse, a 'Stocked Item' may be conceptualised as a 'Part in a Bin in a Warehouse' and where 'Part is in Bin in Warehouse' is a Fact Type Reading of a Fact Type with the name, Stocked Item.
A suitable Fact for that Fact Type may be:
Part, 483, is in Bin, 'A8', in Warehouse, 'Sydney'.
Where the data values, 483, 'A8', and 'Sydney' are the unique identifiers of a part, bin and warehouse, respectively.