The Slash Level Format is a common representation of a character's overall Level used by many players. It shows the level the character has in each class they have held.
For example, using Fire Emblem: Thracia 776's Lara. Say she was promoted from Thief at Level 10, then from Thief Fighter at level 13, then from Dancer at level 12, and now she's a level 18 Thief Fighter. Her overall level in Slash would be shown as 10/13/12/18.
The widespread use of the Slash Level Format has led to a misconception that 2nd Tier characters such as Fire Emblem: The Blazing Blade's Marcus were promoted at a level below 20 in their previous class, despite the fact that there is no evidence that they ever held a class lower than the one they are currently. This has led to people listing levels for them in Slash along the lines of ??/15, supposedly representing the character is level 15 in their current class and reached an unknown level in the previous class, when the character was never actually of the previous class in terms of gameplay.