I do understand that

• CU is released on a monthly basis and contains bug fixes
• PU may be released on a monthly basis and contains security fixes
• Feature pack is released on a yearly basis and contains new features, bug fixes & security fixes

and

• CU should be installed only when it is required
• PU should be installed ASAP
• Feature pack should be installed in a month or two after the release.

Now I have the following queries

• Is January 2018 CU a CU or PU? I has security fix as well.
• What is the recommended frequency for CU installation? I do understand that it should be installed on a demand basis however just trying to see if there are any other recommendations.
• What happens when I skip last 3 CUs and wanted to install the current CU? should I install all the CUs that are missed?
• Can I just keep installing PUs without having to install CUs?
• Finally, what is recommended for Feature Pack installation?