I found this paragraph in the Wikipedian article on Advanced Level but couldn't understand its meaning so please help me.

In the UK it is customary for schools to register with multiple examination boards and to 'mix and match' A-levels to get a combined curriculum that fits the school profile. Schools outside the UK are often unaware that registration with one examination board generally makes registration with them all a 'pro forma' exercise, all A-levels in the British system, being considered exactly equivalent.
Several UK examination boards offer A level qualifications, which are all considered equivalent in prestige/level/quality. UK schools will often register with more than one board so that they offer their pupils a good range of subjects. Schools outside the UK often don't know that once they are registered with one examination board, registering with others is just a formality (formality as a meaning of 'pro-forma' but I find it's use a little unclear here myself).
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My friend is doing A-Level, his school offers A-Level of Camberidge University. Can he also appear in exam of Edexcel? Suppose, he takes Chemistry and Physics of Camberidge's A-Level and Mathematics in Edexcel's A-Level, then will he get one certificate or two separate certificates from Camberidge and Edexcel? Is Edexcel exam conducted by University of London? Can he appear as private student?
These are two different examining boards and will each issue their own certificates. There is no restriction on the number of exams you can enter and you can study as a private student. He needs to get in touch with the examining boards directly to find out how he can do this.
Can he appear in Science subjects like chemistry as a private student? If he can then will he have to arrange those science practicals on his own to prepare for exam?
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Normally, yes. It would depend on the policy of his college or school, they might be willing to help him. This isn't really the best place to get advice on all this, he needs to contact the examining boards and talk to someone at his college.