Google has made some updates to the meta robots directive help documentation, specifically by adding more examples for the max-snippet and specifying for each tag what Google’s default behavior is when the tags are omitted. I highly recommend you re-review these docs starting from the valid indexing & serving directives until the end of that section.

Here are side by side screen shots of the old and the new version if you want to review what changed:

OLD (click to enlarge):

NEW (click to enlarge):

Forum discussion at Twitter.