Definition
The specific FAA-published Airman Certification Standards (ACS) or Practical Test Standards (PTS) document that governs the certificate or rating an applicant is pursuing. The ACS or PTS lists every task, knowledge area, risk-management element, and skill standard the applicant must meet on the practical test, and it is the document the examiner uses to conduct that test. The FAA is progressively replacing PTS documents with ACS documents; the one in force for the applicant's certificate or rating at the time of testing is the 'applicable' one.
Plain English
The official FAA rulebook for the checkride the student is taking. Each certificate and rating has its own version, and the one that matches what the student is testing for is the one that applies.
Context Anchor
Seen when an instructor prepares a student for a practical test or signs a recommendation for that test.
Derivation
Applicable' means 'the one that applies to this situation.' The phrase 'ACS/PTS' is written with a slash because the FAA is in the middle of a transition: older tests use the Practical Test Standards (PTS), newer tests use the Airman Certification Standards (ACS). Saying 'applicable ACS/PTS' is a shorthand for 'whichever of the two documents currently governs the test you're preparing for.'
Why Pilots Care
Using the wrong standards can result in an incomplete or invalid test.
Intuition Check
Do not read applicable as “any ACS or PTS that seems related.” Here it means the specific current FAA standard that controls this particular practical test.
Example Sentence 1
Before signing the endorsement, the instructor reviewed each task in the applicable ACS/PTS to confirm the student met the standard.
Example Sentence 2
Make sure you are studying from the applicable PTS for the certificate being sought.