How to interpret a specific mathematical notation in "FIPS 202" (SHA-3 / Keccak)
It means the length of $Z$ in bits; in other words, squeeze out consecutive $r$-bit blocks from the sponge until you have a $d$-bit string, truncating the last block if necessary.

I expect you may overestimate the number of people who proofread this document with an eye for details like that.  Most cryptographers studying it will have read the Keccak reference, which uses slightly different notation meaning the same thing; most implementors will likely correctly guess that $|Z|$ and $\operatorname{len}(Z)$ mean the same thing when $Z$ is a bit string, or compare to another reference to confirm.