The ISRC is a unique identification system for sound recordings. Sound recordings are not Releases. Formally, a Release is defined as
An abstract entity representing a bundle of one or more Resources compiled by an issuer for the purpose of distribution to individual Consumers, directly or through intermediaries. The Resources in Releases are normally primarily SoundRecordings or music audio-visual recordings. The Release is not itself the item of trade (or Product). Products have more extensive attributes than Releases; one Release may be disseminated in many different Products.
In partner words, Releases are the collection of sound recordings and/or other Resources that can then be productised and made available to consumers under specific terms and conditions (a.k.a. deals). Some Releases may contain only one sound recording. These Releases with just one sound recording come in two flavours: TrackReleases and Single-Resource Releases.
Single-Resource Releases are the digital equivalents of what used to be a "single" in the physical world (although a single typically contained two sound recordings) whereas TrackReleases are Releases that contain one sound recording taken from an album each. For example, when communicating a 10-track album, a typical NewReleaseMessage would contain, besides the Main Release, ten Track Releases (i.e. one for each sound recording that together make up the album).
All Releases need to be identified; this applies to TrackReleases, Single-Resource Releases as well as "normal" Multi-Resource Releases. Typical identifiers for Releases are Global Release Identifiers (GRIDs), UPCs and, in some cases, Proprietary identifiers. One such proprietary identifier used to identify TrackReleases is a concatenation of the main Release's identifier and the ISRC of the sound recording that is contained in that TrackRelease.
However, Releases including Releases that only contain one Resource should not be identified by an ISRC as sound recordings simply aren't Releases.