Elon Musk is a 'special government employee', works for Trump without salary. What does the designation mean?

World's richest man Elon Musk is serving as a 'special government employee', according to White House. The Tesla CEO has an office space in the White House and a government email address. Musk spent over $290 million last year in a bid to assist Donald Trump in the US presidential race against former Vice President, Democrat Kamala Harris.