The County Lieutenants kept full rosters of men on what are known as Permanent Billet Rolls. Most Battalion muster rolls in the Pennsylvania Archives dating to after 1777 are precisely these Permanent Billets. For example, the majority of the Companies listed in the 2nd Series, Volume 14 of the PA Archives (also those rolls from the 5th Series), post-1777 are actually these roll sheets.
When a class was called up, all members of that class were called up to serve throughout the county. For example, if the 1st Class was called, whether you fell under the 1st Battalion or the 7th Battalion, you were required to turn out. New rolls were taken by officers; these are known as Active Duty rolls. Many of these Active Duty rolls for 1777 do not survive.
By the ordinance of the Militia Law, the Class number became the Active Duty Battalion number. Additionally, when a class was called up, the men were assigned a different company number on the Active Duty rolls—the company number they were given corresponded to the battalion they were from on the Permanent Billet—than the company numbers on the inactive duty Permanent Billets.
Pennsylvania was weird and confusing about their militia's.
This means that he was in the "8th class" that would be called up for duty. I would simply state that he served in Capt. Cleders Company, Pennsylvania Militia as a [Rank].