The ICFs can be used by coupling facility LPARs only. ICFs are dedicated or shared. ICFs  can be dedicated to a CF LPAR, or shared by multiple CF LPARs that run on the same  system. However, having an LPAR with dedicated and shared ICFs at the same time is not  possible. With the introduction of Driver 15F (zEC12 and zBC12), the IBM z/Architecture provides a  new thin interrupt class called  Coupling Thin Interrupts . The capabilities that are provided by  hardware, firmware, and software support the generation of coupling-related “thin interrupts”  when the following situations occur: The interrupt causes the receiving partition to be dispatched by an LPAR, if it is not  dispatched. This process allows the request, signal, or request completion to be recognized  and processed in a more timely manner. After the image is dispatched, “poll for work” lo gic in CFCC and z/OS can be used largely as  is to locate and process the work. The new interrupt expedites the redispatching of the  partition. LPAR presents these Coupling Thin Interrupts to the guest partition, so CFCC and z/OS both  require interrupt handler support that can deal with them. CFCC also changes to relinquish  control of the processor when all available pending work is exhausted, or when the LPAR  undispatches it off the shared processor, whichever comes first. A CF image can have one of the following combinations that are defined in the image profile: Shared ICFs add flexibility. However, running only with shared coupling facility PUs (ICFs or  CPs) is not a preferable production configuration. It is preferable for a production CF to  operate by using dedicated ICFs. With CFCC Level 19 (and later; z14 servers run CFCC level  22), Coupling Thin Interrupts are available, and dedicated engines continue to be  recommended to obtain the best coupling facility performance. Coupling Thin Interrupts CF processor combinations GLYPH<SM590000> On the coupling facility (CF) side: – A CF command or a CF signal (arrival of a CF-to-CF duplexing signal) is received by a  shared-engine CF image. – The completion of a CF signal that was prev iously sent by the CF occurs (completion of  a CF-to-CF duplexing signal). GLYPH<SM590000> On the z/OS side:  – CF signal is received by a shared-engine z/OS image (arrival of a List Notification  signal).  – An asynchronous CF operation completes. GLYPH<SM590000> Dedicated ICFs GLYPH<SM590000> Shared ICFs GLYPH<SM590000> Dedicated CPs GLYPH<SM590000> Shared CPs 112  IBM z14 (3906) Technical Guide