Why does File.byLine() return char[] and not string

[Suliman via Digitalmars-d-learn]

On Sunday, 18 October 2015 at 15:40:09 UTC, novice2 wrote:
>> what buffer you are talking.
>
> internal buffer. where result line resides.
>
>
>> And what is "signal"? How it's working?
>
> just the fact for programmer, that result line can be changed 
> by other code (by phobos library code in this case).
>
> no any special programming "signal".

Am I right understand that byLine work like:
read string, put it's to internal buffer, then read new line, 
overwrite existent buffer etc...

byLineCopy is create range that storage all strings in format of 
string, not char?

What is size of this buffer, how it's calculate?