A question-mark, as may be expected, indicates doubt, usually about words which are not found in the OED but whose meaning might be inferred from the context. For example:
mill > ?mill-wheel; ?cogs of the mill (or because the sallow grows by water)
(b) Definitions
An entry in curly brackets should be read as a dictionary definition of the quoted text which cannot be directly fitted into the syntax of the original line. For example:
scrine > {Casket or cabinet for archival papers}
(c) Notes
Entries in round brackets should be read as if they were footnotes, typically giving background information or editorial speculation. For example:
Muse > (The nine Muses are usually represented as the daughters of Jupiter and Mnemosyne (Memory); each goddess presides over an area of the arts and sciences and gives inspiration to its practitioners)
and:
chief > chief, first; best (here Spenser is addressing either Clio, the Muse of history, or Calliope, the Muse of epic poetry; probably Clio. Clio is the first of the nine Muses in Hesiod's Theogony, and is usually represented with an open roll of paper or a chest of books. Spenser calls her "thou eldest Sister of the crew" at TM 53. See 111.5:6- 8, 303.4:6, 706.37:9, 707.1:1)