SciComp is about Computational Science, not Computer Science. Although many Computer Scientists would consider Computational Science to be a sub-discipline of Computer Science, it draws on other fields such as Computer Engineering, Mathematics, Physics, and application domains such as Biology and Chemistry. If a question is about or closely related to the topic of Scientific Computing, it is likelier to receive an expert answer here than in one of the other communities.
Computer science, the topic of those other two sites, is about the theoretical analysis of algorithms and data structures. It could be considered a branch of fundamental mathematics.
Computational science, on the other hand, which is the topic of this site, is about effectively using computers to solve problems in other scientific fields, including physics, chemistry, biology, and even social sciences. Generally speaking, computational science is closer to the "raw numbers" than computer science.