What is the NSFW tag and when we should use it?

I found some post that deserves the NSFW tag, but some post with half-nudity enter in discussion if it is NSFW or not, and sometimes the meaning of the post express a strong feeling or protest.

For example, this post doesn't deserves all the NSFW effects because is a protest and so has strong meaning, and this one.

Some people(or me?) may don't know how to use this feature.

Summarizing, I want to ask you two things:

• Where we should use this feature?
• How it should apply? Should hide all the post or just the image?