In Mail, the Spam folder and Junk folder are distinctly different. Spam folder is in relation to 3rd party software for spam filtering which makes the junk folder obsolete and decommissioned.
ex. see SpamSieve for superior control and spam filtering.
From the Mail Help menu:
Mark messages as junk or not junk
Mail can analyze the content of incoming messages and identify which ones are likely to be junk mail. It highlights these messages in color (by default, in brown) in your Inbox.
If you receive messages that have been highlighted as junk, or messages that are junk but weren’t highlighted, you can mark the messages as not junk or junk. When you mark messages, Mail updates the junk mail database accordingly. Over time, Mail will identify junk mail more accurately.
HideMark messages as junk
To identify one or more messages in your Inbox as junk mail, select the messages and then choose Message > Mark > As Junk Mail, or click the Junk icon in the Mail toolbar.
HideMark messages as not junk
To correct one or more messages that have been misclassified as junk and moved to the Junk mailbox, select the messages and then choose Message > Mark > As Not Junk Mail, or click the Not Junk icon in the Mail toolbar.