University admissions in the UK

I've been trying to understand the semi-centralized application process for British universities, run by an organization called UCAS (Universities & Colleges Admissions Service).

It offers a common application process through which applicants can apply for up to five programs of study, with some restrictions. For example, "If you are applying for dentistry, medicine, veterinary medicine or veterinary science, you can apply for a maximum of four choices in any one of these subjects." And you can apply to only one of Oxford and Cambridge.

Students apply (mostly) by mid January, and Universities make their decisions in a decentralized way, and mostly report them back to students and UCAS by the end of March. Offers of acceptance may be either unconditional or conditional on the student achieving certain grades in upcoming exams.

Students who receive at least one offer can make a firm acceptance, an insurance acceptance, or decline. A firm acceptance is a commitment to attend (and a firm acceptance of a conditional offer is a commitment to attend if the student's grades meet the acceptance conditions). A student may firmly accept only one offer, but if that offer is a conditional one, may additionally choose to accept an additional offer as an insurance choice:
"You can accept an offer as an insurance choice if your firm choice is conditional. Your insurance choice can be conditional (CI) or unconditional (UI) and acts as a back-up to your firm choice, so if you do not meet the conditions for your firm choice but meet the conditions for your insurance choice, you are committed to that course. "
An insurance choice is also a commitment, and so students are advised that they don't need to make one.

Students are given individualized reply dates, depending on when their last decision from their five applications was received. Offers not accepted by the reply date are considered to have been declined.

Students who don't receive any offers or who decline all of those they do receive may apply to courses that still have vacancies, one additional application at a time, through a process called Extra. And, later in the year (in the summer), students who still don't have places, for example because they didn't meet the requirements of a conditional offer, can apply for vacant places through Clearing.

Here's a newspaper account from last year's Clearing process:
"Clearing fortnight is always an emotional rollercoaster. There are, after all, few sleepless nights to rank alongside the one before you get your hands on that ominous bit of paper showing your A-level results. But if you find yourself facing a column of grades lower than you’d expected, all is not lost: of the 413,000 university applicants given firm places last summer, nearly one in 10 (39,000) secured them through Clearing.
Find your course online
Thanks to the wonders of modern technology, finding a last-minute place has never been easier. Whereas applicants would once have spent days trying to secure a berth at university – waiting for individual prospectuses to reach them in the post, then painstakingly wading through in search of viable substitute courses – the internet has revolutionised this process.
Today everything can be sorted out within hours: a visit to the Ucas online Track service will confirm whether you have scraped in, or have indeed lost your place, while the Clearing website offers a handy A-Z of courses looking to fill vacancies. "

This year, early newspaper reports reflect worries that cutbacks may mean that relatively fewer places will be available at Clearing: Students face chaos with no clearing places.