The 100126FC is a buffer from the 100K series, a family of high-speed ECL logic devices designed for clock distribution, signal fan-out, and line driving in systems that need controlled edge rates and minimal propagation delay. It is used in telecom infrastructure, test equipment, and high-frequency data-path designs where the 100K logic family's speed and noise margin are a requirement.
This is typical for legacy ECL logic in hermetic ceramic packages. If your BOM requires lead-free assembly (per EU RoHS or similar regulations), this part will not pass a standard reflow profile without an exemption. For repair, retrofit, or MRO applications where leaded solder is still permitted, the non-compliant status is not a blocker.