diff --git a/documentation/ref-manual/ref-variables.rst b/documentation/ref-manual/ref-variables.rst
index 49e756cfb..c5facfa98 100644
--- a/documentation/ref-manual/ref-variables.rst
+++ b/documentation/ref-manual/ref-variables.rst
@@ -8745,4 +8745,23 @@ system and gives an overview of their function and contents.
 
       The default value of ``XSERVER``, if not specified in the machine
       configuration, is "xserver-xorg xf86-video-fbdev xf86-input-evdev".
-   
+
+   :term:`XZ_THREADS`
+      Specifies the number of parallel threads that should be used when
+      using xz compression.
+
+      By default this scales with core count, but is never set less than 2
+      to ensure that multi-threaded mode is always used so that the output
+      file contents are deterministic. Builds will work with a value of 1
+      but the output will differ compared to the output from the compression
+      generated when more than one thread is used.
+
+      On systems where many tasks run in parallel, setting a limit to this
+      can be helpful in controlling system resource usage.
+
+    :term:`XZ_MEMLIMIT`
+      Specifies the maximum memory the xz compression should use as a percentage
+      of system memory. If unconstrained the xz compressor can use large amounts of
+      memory and become problematic with parallelism elsewhere in the build.
+      "50%" has been found to be a good value.
+