This isn't a "rule" so much as something I learned from several years of refereeing Twilight campaigns.
Everyone wants to be the SAS trooper, the SEAL, the Spetznaz operator.
If you let them, just about everyone in your campaign will be an operator, operating operationally.
But the reality in any military (which is especially what's in play in the original, Cold War-era universe) is that for every Special Operator, there will be 10 grunts and tankers and combat engineers, and 90 truck drivers and computer operators and mechanics and the vast panoply of trades and skills that make up the modern military.
And the fun part of a role playing game is not showing a character's strengths, but seeing how they deal with their weaknesses.
So have used, and propose here, a set of guidelines I used in my various campaigns to encourage characters to generate more a more realistic set of personas for Twilight scenarios.
If a character wants to be:
Any character generated from the "Special Forces" tables: They may only purchase equipment that is rated as "Common", and they may only buy it up to their character's Load Carrying rating. That means a Special Operator can only own what they can physically haul without the aid of a vehicle. By the way - no vehicles.
Any character generated from any Combat Arms tables (Infantry, Armor, Artillery, Engineers, etc): Generated per normal rules, including the ability to acquire vehicles to carry equipment. "Common" and "Very Common" equipment is available at list prices. "Scarce" and "Rare" items are unobtainable.
Character generated from Support arms of service: Support (or non-combat) characters can have access to "Scarce" items at cost, and "rare" items useful to their specialty.