Apposition in Technical Writing

Non-restrictive apposition uses commas on both sides: "Terraform, an infrastructure-as-code tool, lets you define cloud resources declaratively." The appositive phrase "an infrastructure-as-code tool" adds non-essential information about Terraform — it could be removed without changing the sentence's core meaning. Both commas are required: one before and one after the appositive phrase. Option A omits the opening comma. Option C omits the closing comma. Option D omits both commas. In API documentation and release notes, this pattern is the standard way to define tools, libraries, and acronyms inline: "gRPC, a high-performance RPC framework, is used for inter-service communication."

Restrictive apposition (no commas) is used when the appositive is essential to identify which specific person or thing is meant. "The library React" — here "React" identifies which specific library (not just any library). Removing "React" would make the sentence ambiguous. No commas are used because the phrase is essential. Option A ("The developer, Alice") uses non-restrictive apposition — typically when there is only one developer under discussion. Option B is non-restrictive (the colleague has already been introduced). Option D should have commas if PostgreSQL is the only database under discussion. The restrictive vs. non-restrictive distinction affects punctuation and meaning — a critical skill in technical documentation.

Both comma apposition and dash apposition are grammatically correct for defining technical terms inline. Option A (comma) and Option C (em dash) are both valid. However, the question asks which correctly fills the blank — Option C (em dash) is particularly well-suited to technical documentation when the appositive definition is longer or when you want to give the definition more visual prominence. Em dashes are commonly used in API docs and release notes for inline term definitions: "The service returns a JWT — a compact, URL-safe token — for each authenticated session." Option B has the comma misplaced. Option D uses a semicolon, which joins independent clauses, not appositives.

"A security enhancement for OAuth 2.0 public clients" is a non-restrictive appositional phrase — it defines PKCE for readers who may not know the term, placed immediately after the term it defines. This is the standard pattern for technical term definitions in release notes, changelogs, and API documentation: [TERM], [definition as noun phrase]. The phrase is non-restrictive because it does not identify which PKCE — it simply adds definitional information. This pattern is especially valuable in release notes aimed at mixed audiences (engineers and non-engineers): the parenthetical acronym expansion plus the comma apposition provide two layers of definition without requiring a footnote or glossary reference.

Option D is incorrect: "Redis, a key-value store, but it also supports…" — the "but" after the appositive creates a grammatically broken sentence. An appositive phrase is not an independent clause; you cannot follow it with "but + independent clause" as if the apposition were the subject. The correct version would be: "Redis, a key-value store, also supports more complex data structures" (no "but") or "Redis is a key-value store, but it also supports more complex data structures" (apposition removed, coordinating conjunction used to join two clauses). Options A, B, and C are all grammatically correct uses of apposition — restrictive (C) and non-restrictive (A, B).