Did you get Secure Boot turned off now by changing the CSM settings?
was suggesting that you create a 100MB /efi partition in the other thread
. This tells me that Slackware will run with UEFI, but from the conversation I take it that it will not run with Secure Boot. This is not unusual in the present state of affairs for Linux installations, but I am not very familiar with Slackware myself.
So I think you have the choice whether to use UEFI or not, but if you want it then you will need to change the CSM settings, probably for both Legacy and UEFI. Setting for both Legacy and UEFI should also allow you to install Slackware the old-fashioned BIOS/MBR way too.
You may want to make a firm decision about this before you begin... and manually prepare your hard drive for the system that you want. Will Slackware always be your only system? If you think you may dual-boot later with another Linux, I think UEFI/GPT is the best way to go.
If you want to use UEFI, you should prepare the hard disk as a GPT disk (GUID Partition Table), and then setup partitions as you desire... /efi for the bootloader, /home if you want it separate, / for the main system files, and swap.
If you want to use older style Legacy setup, you should prepare the hard disk as "dos" or "msdos" or "mbr" (all the same thing). In this case, you do not need /efi partition, but you can prepare the rest of the drive similar to that above.
If you have Secure Boot now disabled, you can also just throw caution to the wind and let Slackware set itself up however it wants, although I think Slackware does require you to manually partition during the installation steps. But if you don't like it afterwards, blow it away and start again.... no harm to the hard drive to change your mind.