- Not available on that edition (the reason is in the tooltip)
- Depends on the setting, your class or another addon (details in the tooltip)
What it does
Checks which stage your boss mod reports for the current encounter, compared with the stage number you set. Outside an encounter it does not match.
Why you might use it
Compares the stage (phase) your boss mod reports for the current encounter. Use it as a switch condition for a phase-specific rotation, or in a step to hold cooldowns for a burn phase or swap to AoE during an add phase. An encounter with no stage reported counts as stage 1, some intermissions are reported as half stages such as 1.5, and the condition is false outside an encounter. Like the other boss mod conditions it needs DBM or BigWigs, and it is offered on the Classic editions only, not on Retail or WoW Forever.
Example
the boss-mod encounter stage is equal to 2
Settings
What each setting in the condition editor means, from the game's own help.
Operator - How to compare Boss Mod Encounter Stage with the Stage value. This comparison uses basic mathematical comparison operations.
Stage - The stage (phase) your boss mod reports for the current encounter. An encounter whose boss mod has not reported a stage counts as stage 1, and some intermissions are reported as half stages such as 1.5. False when no encounter is in progress; unavailable when no boss mod is enabled or loaded.