The RePC ISO installer wants me to use TMS. Why do I have to do this?

Complicated answer.

The short answer is that (a) you should not attempt to use the ISO installer unless you really need to, e.g. the PC you're booting doesn't support boot from USB, or you're installing on a virtual machine where using an ISO is a lot easier - in all other cases, use the USB stick installer instead, and (b) you must select one the (Re)Install menu options, because the installer will not overwrite any storage without human intervention for safety reasons.

As mentioned above, we have a policy of ensuring that unattended installs must never destroy data (because we don't want lawsuits from customers who trashed vital databases by accidentally PXE booting a TLXOS installer on server hardware, or something similar).  This means that human being must actively choose to destroy on-disk data by some means - you must either use TMS to explicitly give tms_client the green light to overwrite a specific (or auto-selected) disk, or you must choose a non-interactive (Re)Install option from the ISO's boot menu. See the Can I install TLXOS without overwriting pre-existing storage? FAQ answer for more about completing an install using TMS.

We didn't realize for some time that a Reiser4 filesystem can only be safely mounted read-only if the block device identifies itself as inherently read-only, i.e. loopback devices must be set up using the "-r" parameter.  Consequently for several releases we provided "version agnostic" stub ISO installers that didn't actually contain source copies of two of the three essential TLXOS filesystems, and really did need you to use TMS to compete the install.  This was fixed in TLXOS 4.11.0 and later, which are once again capable of performing standalone installs (without kernel panics).

It should also be noted that the RePC ISO installer (aside: ISO is short for "ISO9660", a very specific format - it is not a generic term for an installer!) can only be booted using the Legacy/BIOS method - it is not UEFI-bootable. This means that certain UEFI-only BIOSes (<cough>Intel NUC</cough>) will not be able to boot it.  Since the Maintenance Mode looks exactly like the USB/ISO installer (because it's the exact same code), you might actually be booting the already installed O/S in Maintenance Mode rather than the ISO.  If you do that, there will be no source copy of the O/S from which to install, which will once again result in you being asked to use TMS to complete the install.