The terminology used by different manufacturers and others about how easily DCC can be included in or added to locomotives is not standardized, and has different meaning with different manufacturers. Modelers also use the same terms, but again without a common definition.
In an article in the November/December 2006 issue of N-Scale Magazine, Ron Beardon addresses this issue and proposes a set of Comprehensive Decoder Installation Standards, all relating to the term "DCC Friendly", of which there are definitely different levels of friendliness.
As Ron states in his article the naming system will not always match that of a manufacturer, but if the manufacturers adopt Ron's terms, which are used on this page, modelers would have a better idea of what they are purchasing, or considering to purchase. These definitions are used on the other pages of this web site.
To install any decoder you must remove the shell from the locomotive, and this can sometimes be more difficult that it sounds, but it has no bearing on the definitions.