What is Latency budget?

Example 1.10 (A three-microsecond budget)

A software system must answer within $3\,\text{µ}\mathrm{s}$ wire to wire at the median. The card and kernel-bypass receive path take about $0.9\,\text{µ}\mathrm{s}$ and the transmit path $0.8\,\text{µ}\mathrm{s}$ (the synthetic figures of Figure 1.2); $1.3\,\text{µ}\mathrm{s}$ is left for decode, book, strategy, risk and encode. Split $60\,\mathrm{n}\mathrm{s}$, $120\,\mathrm{n}\mathrm{s}$, $200\,\mathrm{n}\mathrm{s}$, $50\,\mathrm{n}\mathrm{s}$ and the rest in reserve, it fits; the toy book of Figure 1.3 alone, at about $80\,\mathrm{n}\mathrm{s}$ at the median but near a microsecond at p99.9, does not fit a tail target. Chapter 19 builds one that does.