# Under what circumstances does TeX produce a series of tokens with space after a control sequence?

On page 39 of The TeXbook, Knuth said,

Furthermore, spaces are not ignored after control sequences inside a token list; the ignore-space rule applies only in an input file, during the time that strings of characters are being tokenized.

His words confuse me, under what circumstances does TeX produce a series of tokens with space after a control sequence? The output seems like this <cs>, space_10, a_11, 3_12, what input can make TeX produce this output?

\def\macro#1{#1 a3}

When you use \macro\cs then it is expanded to \cs, space, a, 3.