Table:
0
A
B
C
1
document type code
document type description
reference document types
2
CD
b
3
Paper
u
4
Hard Drive
f

Questions and formulas:
Question: all document type codes
Formula: UNIQUE(A1)
Formula2: UNIQUE(A2:A4)
Level: easy
Question: Paper descriptions
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:C1,A1="paper"),2))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:C4,A2:A4="paper"),2))
Level: medium
Question: the document type code
Formula: UNIQUE(A1)
Formula2: UNIQUE(A2:A4)
Level: easy
Question: the description of paper
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:C1,A1="paper"),2))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:C4,A2:A4="paper"),2))
Level: medium