A property can have numerous types of designations or no designation at all. Not having a designation does not mean the property is not historic.
National Register Property is a historic property which can be a site, building, structure or object significant in American history, architecture, engineering, archeology or culture. These properties can be associated with events that have made significant contributions to broad patterns of history, associated with lives of persons significant in our past, embody distinctive characteristics of a type, period, method of construction, work of a master, etc. or have yielded or be likely to reveal important information about our prehistory or history. (Excerpted from the National Register Bulletin.) National Register Properties may qualify for Federal Income Tax exemptions and Collin County exemptions.
A Recorded Texas Historic Landmark (RTHL) is a State designation. You may get more information by reviewing the State of Texas RTHL booklet. Houses with a RTHL potentially qualify for a Collin County tax exemption.
A City of McKinney Historic Marker is a local designation awarded by the City of McKinney Historic Preservation Advisory Board. The owner of the house submits an application along with a documented history of the house and property. If the house is awarded a marker, then the house may qualify for a tax exemption for city and county property taxes.