Does anyone know what the code "F" means in regards to a security clearance?

A code of 'F' on SIDPERS means the person has a F - Interim Secret clearance.
They may retain an F code on ETS if they are in the Guard or Reserve status.
Or on Retirement and placement in an Individual Retired Reserve status (IRR) many service men and women may be subject to recall.
If a person remains in service they will generally be further code-classed.

NOT A COMPLETE LIST OF CLEARANCE CODES:
1C or 2C – Federal
3C or 4C – Federal
5C or 6C – Federal
ADP 1
ADP 2
CIA Lifestyle Polygraph
COMSEC
CONFIDENTIAL
Cosmic Top Secret – ATOMAL/CTSA
DEA Clearance
DISA
DISCO
DOD
DOD Lifestyle Polygraph
DOD Secret
DOD – Top Secret SSBI
DOE – Nuclear
DOE – C
DOE – L
DOE – Q
DOJ – NACI
DOJ – Sensitive
DOJ – Secret
DOJ – Top Secret
EBI – Extended Background Investigation
FAA Clearance
FBI
Foreign Government Information
Industrial Clearance
Inactive Clearance
Inactive Top Secret Crypto
F - Interim Secret
I - Interim Top Secret
X - Secret or Top Secret granted for participation in an incident later being classified
IRS – MBI
ISSA / CISSP
MBI (IRS)
NAC
NASA
NATO
NATO Secret
NSA Lifestyle Polygraph
Position of Public Trust 1C
Position of Public Trust 2C
Position of Public Trust 3C
Position of Public Trust 4C
Position of Public Trust 5C
Position of Public Trust 6C
Polygraph – Counterintelligence
Polygraph – Full Scope / Lifestyle
SAR
S – Secret
Secret COMSEC
Secret SBI
SIDA – Secured Identification Defense Area
SAP – Special Access Programs
SBI – Special Background Investigation
SCI – Special Compartmented Intelligence
SSBI – Single Scope Background Inv.
Technology
TS – Top Secret
Top Secret-Crypto
Top Secret-Crypto SCI / TK / G / HCS-P
Top Secret-Final / Crypto / Polygraph
Top Secret-SCI / SBI
Top Secret-SCI / HSC
Top Secret-SCI / SI / TK
Top Secret-SCI / SI / TK / G / B
Top Secret-SCI / SI / TK / G / HCS
Top Secret-SCI / SIOP
Top Secret-SCI / SSBI
Top Secret-SBI-TS/SBI
Top Secret-CISP-TS/CISP
Top Secret-ISSA-TS/ISSA
Top Secret-SAP-TS/SAP
Top Secret-SCI-CI polygraph-TS/SCI/CI Poly
Top Secret-SCI-Full/Scope Lifestyle-TS/SCI Lifestyle Poly
Top Secret-SCI-TS/SCI
Top Secret-SSBI-TS/SSBI
Top Secret-SCI /SSBI / SI / TK
Top Secret-SCI / TK / G / HCS / CI Poly
Top Secret-SCI / TK / G / Lifestyle Poly
Top Secret-SCI / SSBI / CI Poly
Top Secret-SI / TK / B / +
Top Secret-SSBI / CI Poly
Top Secret-SSBI / SCI / TK / G / HCS
Top Secret-SSBI / Lifestyle Poly
Top Secret-SSBI / SCI
Top Secret-SSBI / SCI/ SI / TK / G
Top Secret-SSBI / SCI / SI / TK / G / B / HCS
Top Secret-SSBI / SCI / TK / HCS
Top Secret-SSBI / SCI-B
Top Secret-EBI
Top Secret-SCI – SI / TK / G / HCS / I / P
Top Secret-CISP – TS / CISP
Top Secret-ISSA – TS / ISSA
Top Secret-SAP – TS / SAP
Top Secret-SCI – CI Poly – TS / SCI / CI Poly
Top Secret-SCI – Full / Scope Lifestyle – TS / SCI Lifestyle
Top Secret-SCI – TS / SCI
Top Secret-SSBI – SCI / TK / B / HCS / CI – Poly
Top Secret-SSBI – TS / SSBI
US Citizen
US Customs
US Treasury Clearance
USPS
VR
Yankee Fire
Yankee White