Personal effects cover is a form or baggage cover (in a similar manner to a travel insurance policy) and this covers a loss of your possessions should the rental vehicle be stolen or broken into.
PEC cover relates to any items normally worn, used or carried by the insured person in daily life, full description can be found in the policy wording and there are stipulations on how these should be stored in the vehicle.
As an additional benefit of a private car rental excess insurance policy, personal effects cover is not often included with supplier offered vehicle rental agreements.
If your question isn't answered here you may want to checkout these pages: