Few problems fall solely into one design discipline’s skillset. The lines between them are blurry. For example, even a basic user interface problem has elements of communication design and experience design. Most designers are comfortable in more than just one area. Just like medical doctors, designers tend to go through some common training (like visual thinking for example), then they hone their skills further and specialize in certain areas over others.
Since you have selected Signs & Symbols, here is some further information on the design skillsets that may be needed in this category.
Communication design is a mixed discipline between design and information-development which is concerned with how media messaging and interventions such as printed, crafted, electronic media or presentations communicate with people. Communication design is not only concerned with developing the message aside from the aesthetics in media, but also with creating and adapting to new media channels to ensure the message reaches the target audience.
Graphic Design is the process of visual communication and problem-solving through the use of typography, photography, iconography and illustration. This design discipline is about creating symbols, images and text to form visual representations of ideas and messages. Common applications include corporate design (logos and branding), editorial design (magazines, newspapers and books), advertising, product packaging, and signage.
Design research is about carefully investigating human experience and behavior to spark and distill insight for teams to address people’s fundamental needs. It involves primary research that is embedded within the process of design, and while it is more likely to occur at the project's inception, it can also be a skillset that is needed across the lifecycle of a design project. Design research is aimed at understanding and improving design outcomes within a process rather than developing domain-specific knowledge within any professional field.