NR 2.10.2 - arabic key signatures

[Federico Bruni]

Hi all

In the following paragraph, taken from 
http://lilypond.org/doc/v2.19/Documentation/notation/arabic-music#arabic-key-signatures

> Other maqams in the same bayati group, as shown in the table below: 
> (bayati, hussaini, saba, and ushaq) can be indicated in the same way. 
> These are all variations of the base and most common maqam in the 
> group, which is bayati. They usually differ from the base maqam in 
> their upper tetrachords, or certain flow details that don’t change 
> their fundamental nature, as siblings.

The last sentence seems quite obscure to me. What are "flow details"? 
What are "siblings" in this context?
I guess that it's about lilypond internals, but the average user (like 
me) reading the Notation Reference is not supposed to know about them, 
right?

I'm trying to translate this stuff but I do not understand it.

Thanks
Federico