Both Opentext and PBS deliver an Archiveserver that is certified for ILM. The archive servers you want to use in combination with SAP ILM RM or RM needs to understand the ILM commandset like: delete an expired file after e.g. 10 years. SAP ILM also uses Webdav as protocol, and not every archive/content server can handle WebDav.
The ILM store is something else. It is also an archive solution, but only offered by SAP. It is a combination of a SAP ILM certfified archive server (based on WebDav) AND a ILM certified storage layer to store the archived in (in this case the SAP IQ database).
So the different is:
Opentext and PBS deliver a SAP ILM certified archive server, but without the storage layer (you have to purchase also SAP ILM aware storage, like EMC Centerra, Netapp Filer, etc).
The SAP ILM Store is a SAP ILM certified archive sever Including ILM aware storage (SAP IQ).
You can use this solution to store both ILM RM archives as well as NLS data from BW.
For more details, see this document: http://sapinsider.wispubs.com/Assets/Articles/2014/October/SPI-archiving-in-the-age-of-big-data
Best regards, Nico Kuijper