need explanation of how "mode 4/9 replace block" works

[SmayK]

As stated in the title, after several searches, I haven't found how to use "mode 4/9 replace block."
I know how to select the area, choose the block to place, and finally, I think it's okay (which is identical to "mode 3/9 fill," which I know I can use without any problems). When I click my key to execute, nothing happens.
Thank you for your help.

[maruohon]

In the Replace mode you need to additionally select the block to replace, which is by default done with Shift + middle click. The two modifier hotkeys for these are called toolSelectModifierBlock1 and toolSelectModifierBlock2.