There are two types of merge tags, Person & Job. The Person tags are available everywhere in the system when composing an email or SMS. Job tags are only available when emailing or texting within a job. Job tags can only be used from within a job so the system knows which job detail information to use, outside of a job the merge tags will not be available.
Person Email Merge Tags: You can pull in all of the standard fields that are available in Loxo, as well as any custom columns (minus hierarchy or tag columns).
Job Email Merge Tags: Job Company, Link and Description are available from within a job
To add custom merge tags, you simply will need to build additional fields in Loxo to capture this information. Check out this article to learn how: http://help.loxo.co/how-to-s/how-do-i-customize-my-people-company-grids