In this case: 

 UCSR0A = (1 << U2X0);

`1<<U2XO = 00000010`, `1` is shifted to left to U2XO position so when you give this value to UCSR0A the reister's content will be:

 UCSR0A = 00000010

This operation will clear all bit but the U2X0, for example if RXC0 would have been set to one, then this operation would clear it. If you are not aware of this that could cause some headache later.


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In this one:

 UCSR0A = UCSR0A | (1 << U2X0);

which is equal to this:

 UCSR0A |= (1<<U2X0);

Here you set only the second bit to one and leave the rest unchanged which is preferable, because you won't change something accidentally. The content will be:

 // UCSRA0 = xxxxxxxx before operation something is in the register
 UCSR0A = UCSR0A | (1 << U2X0); // OR current with 00000010
 // UCSRA0 = xxxxxx1x something OR 1 will be 1

where `x` is the unchanged/previous value of the bit. If `x` is 1, **1 OR 0** will remain **1** and if `x` is 0, **0 OR 0** will remain **0**.


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So I suggest the second way, because that way it is easier to track which bit is set and which not and you won't change previously applied configuration by mistake.