What is the difference between assertive and non-assertive words? 

I haven't been able to find an answer in my online linguistics sources such as the SIL Glossary of Linguistics Terms. 

The only source I could find was an English Grammar site, namely <http://www.englishgrammar.org/nonassertive-words/#mjsDTl7shKvqoAB0.99>

This source states that assertive words are generally used in declarative sentences. 

Examples of assertive words include "some, once, already, somebody, something, sometimes, somewhere, someone."

The source also explains that non-assertive words are used " in questions and negatives. They are also used in if-clauses and with adverbs, adjectives, verbs, prepositions and determiners that have a negative meaning." 

Examples of non-assertive words include "any, anything, anybody, ever, & yet." 

However, as I read the source, I wasn't able to determine exactly what the semantic or pragmatic difference between assertive and non-assertive words is. I'm hoping that someone can spell this out for me, and/or recommend a source that explains the difference. 

Also, the link provides an example is given of an assertive word used in a question, namely "Did you want something? (Suggests ‘I think you want something’."