Could somebody elaborate 'use a const pointer when the non-optional form would have used a reference'?

in cppguide.html,
Could somebody please elaborate 'use a const pointer when the non-optional form would have used a reference'? It will be also helpful if some examples can be added.

My understanding is as follows,
* Non-optional input parameters - values or const references
* Non-optional output and input/output parameters - references
* Optional input parameters(by value) - absl::optional
* the non-optional form would have used a reference(**<= This means Optional input parameters by references?**) - a const pointer
* Optional outputs and optional input/output parameters - non-const pointers

Thanks in advance.

> Parameters are either inputs to the function, outputs from the function, or both. Non-optional input parameters should usually be values or const references, while non-optional output and input/output parameters should usually be references (which cannot be null). Generally, use absl::optional to represent optional by-value inputs, and use a const pointer when the non-optional form would have used a reference. Use non-const pointers to represent optional outputs and optional input/output parameters.

Start in cppguide.html by reading the surrounding parameter guidance and the quoted rule about const pointers. Clarify when an optional input should use a const pointer rather than a reference, and add examples that distinguish the listed parameter cases. Done means the wording and examples make the rule unambiguous.

Written by the indexing model from the issue text.