These are two different concepts:
ISO 11137-2, section 4.3.4: "A simulated product shall only represent a product family if it constitutes an equivalent or greater challenge to the sterilization process than that provided by members of the product family"
ISO 11137-2, section 5.2, Sample item portion (SIP) : "For product with an average bioburden greater than or equal to 1.0, whenever practicable, an entire product (SIP equal to 1.0) should be used for testing in accordance with Table 1. When the use of an entire product is not practicable, a selected portion of product (SIP) may be substituted."
So whenever you cannot test a product as a whole, but only parts of it, then you talk about SIP. Typically, SIP applies to actual product in case you have designed your simulated product small enough to be tested as a whole. But theoretically you could also test a portion of your simulated product if otherwise not practicable.
Hope this helps,
