Crystals over 30 MHz (approximately >200 MHz) are normally operated at collection resonance wherever the impedance seems at its minimum and equal towards the series resistance. For these crystals the collection resistance is specified ( Predecessor of surface mount technology but still valuable in sure eventualities, such as prototyping by https://bookmark-group.com/story4580946/helping-the-others-realize-the-advantages-of-smd-crystal-oscillators