The -a 9 association attack tries each word in a single wordlist against a single hash. It is used when a likely password or password component is already known, correlated with each target hash. It is similar to John the Ripper's “single” mode.

• The wordlist must be exactly the same length as the target hash list.
• If the target list has work factors, they must all be identical. For example, all bcrypts can be cost 10, but a mix of cost 10 and 11 will not work.

• Remember that -a 9 can be used with rules! And if you do this with slow hashes, you probably also want to use -S.
• You can also use a directory of wordlists, as usual.

This forum post goes into more details:

https://hashcat.net/forum/thread-9534.html