This symbol indicates that the trademark has been registered in the relevant trademark registration agency and is protected by law. In STIHL's products, the circled "R" is usually used to indicate that the trademark has been registered and belongs to a trademark protected by law.
It should be noted that different regions and countries may have different regulations and standards for trademark registration and the use of trademark symbols, so you should abide by local laws, regulations and norms when using trademark symbols. If you have specific questions about trademark registration and the use of trademark symbols, I suggest you consult a professional trademark registration agency or lawyer.