Final answer:
The ISG relationship code that denotes an item as only usable in place of another under particular circumstances is "Conditional" (C). This code implies there are specific conditions that determine when the item can be used as a substitute.
Step-by-step explanation:
The ISG (Item Standardization Guide) relationship code that identifies an item that can only be used in place of another for a particular use or under certain conditions is C) "Conditional".
In practical terms, a conditional code indicates that the item in question can serve as a replacement to another item, but such use is limited by specific conditions. These conditions may relate to the operational environment, performance requirements, or other unique factors defining the suitability of the substitute.
This contrasts with an Equivalent (A) code, which implies that items can be used interchangeably under any conditions, and a Substitute (B) code, where the replacement can be used universally, without restrictions. A Restricted (D) code would typically indicate that there are limitations on the use of the item, but it does not specifically relate to its substitution properties.