How do I know which letter to use next to my section number?

The different letters represent where the row breaks are. Depending on the row, the ticket pricing is typically different:

Section 101: Rows 10-39 are "L" and 40-68 are "U"

Section 102: Rows 10-39 are "L" and 40-64 are "U"

Section 103: Rows 10-39 are "L and 40-60 are "U"

Section 104: Rows 6-34 are "A" and 35-50 are "B"

Section 116: Rows 6-34 are "A" and 35-46 are "B"

Section 122: Rows 6-34 are "A" and 35-38 are "B"

Section 134: Rows 6-34 are "A" and 35-50 are "B"

Sections 301 - 310 and 313 - 322: Rows 1-12 are "L" and 13 & up are "U"

Sections 311-312 and 339-340: Rows 1-16 are "L" and 17 & up are "U"

Sections 323 - 328: Rows 1-16 are "L" and 17 & up are "U"

Sections 329 - 338 and 341 - 350: Rows 1-12 are "L" and 13 & up are "U"