Could be Pulse Repetition Frequency
Actually it is a bit more mundane, it is PReFerred, as in the Preferred number. It replaces an entire list of previous prefix's to military numbers such as C for Connector, R for Resistor, W for Wire, etc.
An example:
MIL-PRF-27/356C succeeds MIL-T-27/356B and is the specification for:
Transformers AND INDUCTORS (AUDIO, POWER AND HIGH-POWER PULSE), INDUCTORS, POWER, TF5S04ZZ
Performance, If it's mil-dtl, it's detail
It's the standard oil used by the military for Gas Turbine Engines typically found in aircraft.
Military Performance Specification 81733 Revision D
It is the Military Preference 23699. Just a type of turbine engine oil. What the actual numbers mean I am not sure. Just numbers I believe. as all types of Oils and preservatives in the military carry a MIL spec but can be made by different manufactures but have to meet that specific standard.
Answer: PRF (Personnel Readiness File/Folder) files are used in technical application and are not likely to be accessed by your average PC user. In general, PRF files are Windows system files, although they have other technical applications as well.
.mil stands for military. It Is The Highly trusted domain.
It means specifications.
.mil stands for military top-level domain name
Pro-Seal 870 is a Corrosion Inhibitive Sealant built to meet MIL-PRF-81733D Classification. It is a is a two-part, manganese dioxide cured polysulfide compound. It's also known as BMS 5-95
The prf means the coin is a proof, proof coins are not intended for circulation and have mirror like fields (spaces where there is no design) and generally frosty designs. US coins are graded on a scale from 1-70 with 1 being barely readable and 70 being absolute perfection. Prf 65 is a fairly low grade for a US proof made recently. For older coins Prf 65-66 are typical due to damage from improper storage. But for newer proofs, prf 68 is low and 69 and 70s are common.
Pulse Repetition Frequency.
SGLI