HUD | Citadel docs

player.ui.actionbar("Bet 50 · Chips 1200", "light_purple")

the line above the hotbar

local bar = ui.bossbar{ title = "Race starts in 30 s", color = "purple", progress = 1 }

bar.title, bar.progress, bar.color, bar.visible

live; progress 0..1

bar.visible = false

off every screen; true or nil shows it again a bar's colour is its own closed set: pink, blue, red, green, yellow, purple, white. It is not a colour value and has no hex form; anything else is white.

bar:destroy()

local mine = player.ui.bossbar{ title = "Your horse: Lightning" }

one player's bar, the same handle

ui.sidebar{ title = "INDIGO", lines = { "Chips 1200", "Bet 50", "", "Roulette" } }

everyone in the room

player.ui.sidebar{ title = "INDIGO", lines = { ... } }

one player; lines run top to bottom

ui.sidebar(false)

removes the sidebar, as nil does; player.ui.sidebar(false) for one player

ui.tab{ header = "INDIGO", footer = "minefort.com" }

every title, line, header and footer here is text: { "Chips ", { "1200", color = "gold" } }

A boss bar re-sends on every change. A late joiner sees it at the next change.
1.8 has no boss bars. A 1.8 client shows the title on its action bar.
The sidebar sends only changed lines. Repeated lines are fine. Long or extra lines are trimmed.

A canvas on the screen

player.ui.overlay puts a 128×128 canvas in a corner of one player’s screen. It draws like a canvas. Only that player sees it.

local map = player.ui.overlay{ corner = "top_right", size = 96, margin = 4 }   -- nil where player:supports("overlay") is false
if map then
    map:clear()                                  -- see-through: the game shows through
    map:rect(0, 1, 128, 127, "#101820", 10)      -- rows 1..127; row 0 is the engine's and never shows
    map:text(8, 6, "MAP", colors.white, 12)
    map.open                                     -- false after destroy or when the player leaves
    map:destroy()
end

corner is top_left, top_right, bottom_left or bottom_right. size is screen pixels a side at GUI scale 1 (16 to 511). margin is the gap to the edges (0 to 63). The client’s automatic GUI scale multiplies both.
It stays over room moves and respawns. It goes when the player leaves.
Changed pixels go to that player once per tick.
Only 26.3 draws it, with the engine’s shader in the resource pack. A pack that names player.ui.overlay makes the pack required for 26.3. A player who declines it is disconnected with a how-to message.
Several at once are fine: each is its own canvas.
Shader mods (Iris, OptiFine) replace the shader. There the HUD does not show.