PxrSeExpr is deprecated in 24.0 because of the loss of optimized networks that occur if you embed a C++ node in the middle of OSL nodes. It should be replaced with custom OSL snippets or simple arithmetic nodes.
Because it is sometimes useful for an artist to be able to experiment with a procedural materials and patterns, rather than relying on painting textures or built-in nodes, PRMan includes this SeExpr node. Based on Disney Animations's open source SeExpr system, this node is a scriptable pattern generator and combiner. It is unique among the factory plugins in its "dynamic binding" capabilities. Pattern network authors can wire upstream nodes into arbitrary (named) connection points and these can be referred to in the SeExpr script.