飛機客艙座位之分類(B、S、H、V、L、M、U、V艙)

These are called the sub-class, they represent the different ticket condition and regulation with the Economy Class...
Usually, these different sub-class would have different condition and regulation... And as the ticket is cheaper, the more condition and restriction there would be like you said limiting the flight you are allow to take (eg. early bird flight), restriction on maximum staying time, date of flying (eg. Monday-Thursday flight only), non-refundable, no change of date etc...
But if your ticket is more expensive, usually it would allow you to fly when you want on the route with more or less no condition and restrictions apart from maximum staying time being 1year and allow to fly when you wish if there is seat available on that flight...
For those not allow to fly on flight 9051-9100/9401-9450/9601-9700... Its usually because these are code-share or charter flights... For code-share, usually you would need a higher sub-class ticket (more expensive ones) to fly on them and if its charter flights, then that means these flights can only be booked via travel agency as its being chartered by them...

They represent the sub-class in each First/Business/Economy class...
Different sub-class have different price/condition/restriction...
Usually the lower the class (Cheaper ticket), there would be more restriction like Monday-Thursday only (As Friday to Sunday demand is higher), no change of date/time, no refund if cancellation, shorter maximum staying time etc... And as the ticket is more expensive, there will be less restriction/condition...
Like if you get a Full fare Economy class ticket which is Y class, your ticket will have more or less no restriction on changing date/time... But 1 restriction is that it will be within 1 year...

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