Queries on Archiving Info structure

Hi,

In Archiving i have noticed that an Archiving object has one or more infostructures available and some objects has no infostructures at all.

1.How to identify the Info structures from multiple for Activation?

2.If i activate two info structures for an object, wat happens?

3.Table AIND_STR1 has infostructures for almost all objects but when i enter the same info structure in sarj it says no info structure found.

Ex: MM_EINA(Purchase info records) has no info structure avialbele in sarj, but there exists an info structure (SAP_MM_EINA_AS) in the table AIND_STR1 for the same object, how to use this for archiving?

Kindly clarify the doubts.

regards,

Mans.