I understand that if a plan invests in Employer real property it must be QERP and subject to certain restrictions. But, if the plan is investing in real estate does that need to meet the diversifcation req't, no more than 10% ? is there a distinction between any real estate or real estate that is to be QERP?

only employer RE is subject to 10% limit. Non er re is subject to diversification requirements of ERISA, i.e., must be prudent investment under plan's investment policy.