Specialists carry responsibility for a certain topic. They keep track of issues in this topic and/or spend the majority of their time there. Sometimes there is a lead in this topic that they report to. You can be a specialist in only one topic. The specialist description is a paragraph in the job description for a certain title. A specialist is listed after a title, for example: Developer, database specialist (do not shorten it to Developer, database). The if you can have multiple ones and/or if you don't spend the majority of your time there it is probably an expertise. Since a specialist has the same job description as others with the title they have the same career path and compensation.