What does "six-digits" mean?

[Thunu]

Business English Lesson, Advanced Level

ESL/EFL Test #95 "Corporate Policy (3)", question 3

Our ......... income is at least six-digits.

(a) annual
(b) allocated
(c) proverbial
(d) year

Business English Lesson, Advanced Level

ESL/EFL Test #95 "Corporate Policy (3)", answer 3

Our annual income is at least six-digits.

Correct answer: (a) annual
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Please help me: What does "six-digits" mean?

Thank you
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Reach an agreement as ploughing, don't let argument happen at crops.

[Mister Micawber]

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The Arabic numeral system consists of 10 digits: 1,2,3,4,5,6,7,8,9 and 0. A six-digit number has any combination of six of these digits, so here are some six-digit numbers: 436,912; 890,000; 720,301.
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Native English teacher at Mister Micawber's