Hi Costa,
This option is not under user project but user workspace .metadata folder.
We can find it under path:
{User workspace}\.metadata\.plugins\org.eclipse.core.resources\.projects\{Your_Project_Name}\.i
ndexes\properties.index
This file is not a full readable file, but we can still get information of it with EltraEdit. For example, this is one properties.index file for one of my project "MKV30F64xxx10_Project1"

Here id "com.crt.advproject.config.exe.debug.433576328.510735946" is defined in your project.cproject file. Just perform search in .cproject, we will find:

This is the right configuration I defined in my project

Because properties.index file is under workspace folder but not in your project folder, when you move your project to another workspace, the active build configuration wouldn't keep same.
Have a great day,
Jennie Zhang
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