Reference properties in a Content Type model should  use the syntax:

type: reference:TourGuide

instead of 

type: ct:TourGuide

as it is currently implemented.

Reasoning:

The intention of this ticket is to make the content model very clear to developers. The important thing is that a developer (and other stake holders in the project) should know when a field is a reference, as this will impact the UI, the templating, configuring REST endpoints and working with REST responses.

The word "reference" is a good choice for this as it is easy to read, it is plain english, and it is consistant with how most CMS call reference fields. (Please see the table at the top of this page: https://wiki.magnolia-cms.com/display/TH2/Competitor+Research+-+Content+types )  

I appreciate the reasoning mentioned in the review session that 'ct' is much shorter than 'reference': We do strive to make a compact content type definition. But in this case I think the benefit of a shorter syntax is outweighed by readability, ease of comprehension, and familiarity.