Rules, Elements, and Options are documented in ref/Import and you can find all of the details I summed up in tutorial/ImportingAndExporting (including the esoteric "Options" examples). There is a ton of information in our documentation and we do our best to both have good coverage in documentation and to assist users in finding the specific info they're looking for quickly.

Hi Mike, hopefully I can help clear up a few things. I go into detail about these "Framework" elements i.e. elements that are known by every single format in this post: https://mathematica.stackexchange.com/a/212034/38159. I will reiterate the most important points here:

• The Import framework has a few elements that it knows how to do for every format: All, "Rules", "Options", "Elements".
• Elements gives the list of elements that are supported for a format.
• Rules is a list of rules with elementName -> data for every element.
• All is a list of the data without the element name.
• "Options" allows you to specify at registration a list of elements that would be returned if you for "Options", importing that subset of elements. This is a little confusing and not often practical. See the MSE post I linked for a small example of how this works if you are still curious.
• There is currently no way to get a list of options that a format accepts, maybe in future versions.

As for your valid suggestions for CSV, you are correct there is not a way to give columns special handling at the time of import and these are great ideas that I've been thinking about for quite some time. Hopefully in future versions CSV can better deal with typed columns more efficiently and with some better direction. Please feel free to keep posting your questions on community as you continue to learn Wolfram Language!

Hi Mike,

For CSV the options are documented here, for Excel XLS they are documented here and for XLSX, here.

Is there a specific problem with Import that you are trying to solve?