Central Excise rules

Can any one explain me the Rule 7 of Cenvat Credit Rules, 2004.

In this rule there is the input service provider which has both types of units, one producing exempted and another unit producing dutiable goods. In this case units raised bills in the name of ISP and ISP has distributed the input to its dutiable units as per his discretion and not distributed to exempted units because as the their final product/ service is expempted they can not claim the input of such goods.

Say for e.g. the ISP has two unit of each type i.e. one exempted and one dutiable units. Both units raised bills in the name of ISP having input credit of Rs.500 & Rs.1000 respectively. Now my Question is that how the ISP distribute the such input of Rs.1500 in total to their units.

- Whether it can distribute Rs.1000 as Rs.500 arise from expempted unit hence, redundant.

- Whether it can distribute Rs.1500 in full to dutiable units?

i have read in rule 11 of central excise rule in which contain the invoice shall be serially numberd and shall contain the registration number,address of the concerned central excise division,name of consignee,descripttion ,classification , time and date of removal,mode of transport and vehicle registration number,rate of duty, quantity and value of goods and the duty payable thereon.but in case of one manufacturing company duty payable,excise registration number etc are not shown in invoice.whether this invoice is valid or not. But when i discussed this case with my senior bosss he says when company transfer his stock to godwon invoice issued under rule 11 of central excise rules,but when finished goods sold from godwon then invoice not issued as per rule 11 of central excise rules.