What is a product attribute?

A product attribute is a classification or descriptive feature or attribute of a product. Things like colors, difficulty levels, sizes, length of experience, etc. are all good examples of product attributes.

Does a product have to be assigned an attribute?

No. While it is recommended that you add in attributes so that users can more easily filter down to the results they desire it is not required. 

Is there a limit to how many product attributes a product can be assigned?

No, a product can have as many attributes as you wish. 

How does a category differ from a product attribute?

A category is a broad categorization of products. While product attributes can also fall into this classification, product attributes are more intended to describe features or attributes of various products. 

An example is with Rafting activities. You might categorize your rafting activities under a "Rafting" category (to differentiate them from Kayak or Paddleboard activities), but then may assign things like difficulty levels (Category 1-5), seasons (best in the spring for winter run-off), or things like "Adults Only Experience" to your activities as various product attributes.