In many places I see referenced a "+1 rule" for characters legal in Adventurer's League play. What does this mean?
Version 1.1 Player's Guides
The “+1 rule” is shorthand for the restriction on being able to use the Player's Handbook plus one extra source to build your characters for the Forgotten Realms campaign.
WoTC has two different campaigns that utilize different limitations (currently v1.1).
Forgotten Realms maintains the "+1" rule (see below), while Eberron does not and includes additional resources and limitations.
Forgotten Realms (Waterdeep/Dragon Heist)
Your 1st-level character is created using any race and class options in the Player's Handbook plus one other resource from those listed below (The "PHB+1 rule").
- Elemental Evil Player's Companion (EEPC)
- Sword Coast Adventurer's Guide (SCAG)
- Volo's Guide to Monsters (VGM)
- Xanathar's Guide to everything (XGE) [includes Tortle package]
- Mordenkainen's Tome of Foes (ToF) [Chapters 1-5 only]
The variant or optional rules are available when creating your character:
- Variant Human Traits (PHB)
- Half-elf and Tiefling Variants (SCAG/ToF)
- Option: Human Languages (SCAG) [This option isn't subject to PHB+1]
- Blessing of Corellon (ToF) [This option isn't subject to PHB+1]
Eberron (There is no +1 rule per the ALPG linked above.)
Your 1st-level character is created using any race and class options in [see below]. No other resources are available - even if other campaign documentation (i.e., certs) otherwise allow it.
- Elemental Evil Player’s Companion (EEPC)
- Sword Coast Adventurer’s Guide (SCAG)
- Volo’s Guide to Monsters (Bugbears, goblins, and hobgoblins) (VGM)
- Xanathar’s Guide to Everything (XGE)
- Wayfinder's Guide to Eberron