An NDA needs to specify what types of information are protected. Usually this is done in a lengthy definition of what is included and excluded in “Confidential Information”. You can choose either a narrow or broad definition of what is protected. The other obligations under your NDA will then only apply to the type of information you specify.
You might decide to limit the obligations under your NDA only to written documents marked or stamped as “CONFIDENTIAL”. This gives you the benefit of being quite clear about what information of the other party’s you need to worry about protecting. For example, it is the stack of documents with the red confidential stamp on them only. You can separate those from the other documents and treat them more carefully.
Or, you might go a little further than this, and accept that you might reveal some confidential information in an oral conversation, so the protected information should go wider than just written documents. But maybe it only extends to oral information which is stated to be confidential at the time it is disclosed. This still allows you to manage oral information in a similar way to the stamped confidential documents.
Other people worry less about managing the confidential information of the other party, and worry more about what happens if they forget to stamp (or specifically state) that their own information is confidential when disclosing it.
If you take this approach, you might favour a broad definition that certainly includes information marked or stated to be confidential, but also includes any other information that is reasonably intended to be confidential. That is, if I reveal to you my secret herbs and spices, that information should still be protected, even if it is not marked or stamped as confidential.