A semiotic system is causally affected by signs in action (semiosis).
Semiotic systems can be regarded as the embodiment of semiotic processes (Peirce CP 5.314).
Surely, this blurs a distinction between agents and processes. Peirce was a process thinker, i.e., a representative of a philosophical tendency of treating processes as being more fundamental than entities as metaphysical and epistemic categories. A process philosophy can address entities as stable bunches of processes. A semiotic system can be understood in these terms as a stable (both spatially and temporally) cluster of semiotic processes, or semiosis.