What are the strings in Curly brackets {...}?

l10n
#1

Hello.

I'm reviewing the Arabic translations.
What are exactly the {} brackets?
Like in:
"
The file [2][3] is being held in use {by the following process: Name: [4],
ID: [5], Window Title: [6]}. Close that application and retry.
"
And:
"
The Windows Installer service cannot update the system file [2] because the
file is protected by Windows. You may need to update your operating system
for this program to work correctly. {{Package version: [3], OS Protected
version: [4]}}
"

Can I add another {} brackets in the string if I need them?

Regards,
Safa.

#2

"Safa Alfulaij":

What are exactly the {} brackets?

They mark the part of the string which can be omitted if there is no [parameters] value available inside them.