diff --git a/VARIABLES.md b/VARIABLES.md
index 3d82becf..e9812c14 100644
--- a/VARIABLES.md
+++ b/VARIABLES.md
@@ -78,8 +78,8 @@ These variables should be set when tests are scheduled (when running `isos post`
 
 | Variable | Explanation |
 | -------- | ----------- |
-| `ISO` | contains filename of ISO that is used for booting, if ISO is already downloaded |
-| `ISOURL` | contains URL for ISO to boot, openQA will download it. Set this or `ISO`, not both |
+| `ISO` | contains filename of ISO that is used for booting. If ISOURL is not set, file must exist in /var/lib/openqa/share/factory/iso. If ISOURL is set, it will be downloaded to this name instead of its original name |
+| `ISOURL` | contains URL for ISO to boot, openQA will download it |
 | `DISTRI` | contains distribution name (should be same as in WebUI, probably `fedora`) |
 | `VERSION` | contains version of distribution |
 | `FLAVOR` | indicates what type of distribution is used. Three Pungi properties, joined with `-`: `variant`, `type`, and `format`. e.g.: `Server-dvd-iso`. Special value `universal` is used to schedule the group of tests that should be run once each per arch per compose, against the 'best' available ISO |