Creditable Service refers to the time that an employee has served at a specific job not within one company, but also within multiple companies. 

E.g. An employee is currently working in company ABC as a Software Engineer for 1 year. He also had worked in company DEF as a Software Engineer for 2 Years before joining the current company. As he had worked in both companies as a Software Engineer, his creditable service as a software engineer is 3 years. 

Service Period (YOS) in your OrangeHRM refers to the time that you have worked at the current organization regardless of the the job title.