Bound for string types (null for non-string or when unbounded → MAX/TEXT).
Concrete PostgreSQL column type, e.g. VARCHAR(255), BIGINT, TIMESTAMPTZ.
Schema-builder SchemaFieldType family ('string' | 'number' | 'boolean' | 'datetime').
Concrete SQL Server column type, e.g. NVARCHAR(255), BIGINT, DATETIMEOFFSET.
The schema-builder field-type family + the concrete per-dialect SQL types we infer.