Once checkboxes next to a few project attributes are marked, options for merging and replacing them with a global attribute will show up. The attributes of the same type can be selected only. To replace the selected attributes with a global attribute, you'd need to choose the attribute you want to replace them with from the list of all global attributes with the same type.
In case of replacing attributes of the "Options" type, you'd be prompted to map the values of selected project attributes with the values of a global attribute that they will be replaced with or you can choose not to map some attributes' values and in this case, such values will be lost.
After replacing the selected project attributes with a global attribute, this attribute will be added to all the projects where the selected attributes have been used. Initial project attributes will be deleted.
When merging several project attributes into a new global attribute, there are options for specifying the new attribute's name and enable/disable the "Required attribute" option.
In the case of merging attributes of the "Options" type, there is a way to add new values, edit the existing ones, rearrange them, and choose the default value. Once the project attributes are merged, they'll turn into a global attribute.