I seems they mean: all the buildings that give a town character, the typical elements that make you recognize a town from far... A church, palace etc.

Taken from my GPS application, see for example:
http://www.architecturaldigest.com/gallery/landmark-architec...
https://en.wikipedia.org/wiki/Landmark

Venu de l'allemand ''signalarchitekture'' peut-être: Neubau oder zumindest ein bauliche Erweiterung des Wien Museums für die nächste Legislaturperiode des Wiener Landtags angekündigt. Museumsdirektor Wolfgang Kos (vgl. Interview im Standard, 3. 9.) ist begeistert und wünscht sich "Signalarchitektur" mit "Landmark-Charakter" -

https://www.modul.ac.at/uploads/files/.../Thesis-2011-Wachlo...
Evolved Communication Model for Interactive Media Façades. 75. 8.3. ..... needs. The efforts made here are referred to as the so called “signal architecture”.
At a first glance, it seems that the media façade originates from classical signal
architecture. Signal architecture mainly aims to (Schediwy, 2009)
 attract the public audience, even from a larger distance
 generate a landmark, which is easy to distinguish from the surrounding area
 foster a certain identity
 Communicate/narrate certain intrinsic functionalities


fields.eca.ac.uk/deaua/wp-content/uploads/2008/10/chapter8.pdf
by business, and the open differentiation of urban space in the city as a whole2 – and it ... Skateboarding counters signal architecture with a body-centric and ...