XPath provides the ability to compare strings with expressions. However, if you try to compare strings with lt or gt with DPath, you get the following error:

[error] Test edu.illinois.ncsa.daffodil.section23.dfdl_expressions.TestDFDLExpressions.test_comparison_operators_26 failed: java.lang.Exception: Schema Definition Error: Operands for operator 'lt' must have numeric type. Types were: String and String.

See tests:
test_comparison_operators_23
test_comparison_operators_24
test_comparison_operators_25
test_comparison_operators_26

in the following files:
daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section23/dfdl_expressions/expressions.tdml
daffodil-test/src/test/scala-debug/edu/illinois/ncsa/daffodil/section23/dfdl_expressions/TestDFDLExpressionsDebug.scala
daffodil-test/src/test/scala/edu/illinois/ncsa/daffodil/section23/dfdl_expressions/TestDFDLExpressions.scala