Complex sentence: About half of the SES is designated "Career Reserved", which can only be filled by career employees, with the other half designated "General", which can be filled by either career employees or political appointments as desired by the administration.
Split into simpler sentences: About half of the SES is designated "Career Reserved", which can only be filled by career employees. The other half is designated "General", which can be filled by either career employees or political appointments as desired by the administration.