A Study of BGB-16673 Compared to Investigator's Choice in Participants With Chronic Lymphocytic Leukemia or Small Lymphocytic Lymphoma Previously Exposed to Both Bruton Tyrosine Kinase (BTK) and B-cell Leukemia/Lymphoma 2 Protein (BCL2) Inhibitors

PFS is defined as time from the date of randomization to the date of first disease progression or death, whichever occurs first, as determined by IRC using modified 2018 International Workshop on Chronic Lymphocytic Leukemia (iwCLL) criteria for participants with chronic lymphocytic leukemia (CLL) and Lugano classification for participants with small lymphocytic lymphoma (SLL).

OS is defined as time from the date of randomization to the date of death due to any cause.

PFS is defined as time from the date of randomization to the date of first disease progression or death, whichever occurs first, as determined by IRC using modified 2018 iwCLL for participants with prior exposure to ncBTKi.

PFS is defined as time from the date of randomization to the date of first disease progression or death, whichever occurs first, as determined by the investigator using modified 2018 iwCLL criteria for participants with CLL and Lugano classification for participants with SLL

ORR is defined as the percentage of participants with best overall response of complete response (CR), complete response with incomplete bone marrow recovery (Cri), nodular partial remission (nPR), or partial response (PR) as assessed by the IRC or by the investigator.

Rate of PR-L or higher is defined as the percentage of participants with a best overall response of CR, CRi, nPR, PR, or PR-L as assessed by the IRC or by the Investigator

DOR is defined as the time from initial response to disease progression or death, whichever occurs first, as assessed by the IRC or by the investigator.

TTNT is defined as the time from the date of randomization to the date of next anti-CLL/SLL treatment.

Safety will be assessed by monitoring and recording of all treatment emergent adverse events (AEs) graded by National Cancer Institute-Common Terminology Criteria for Adverse Events (NCI-CTCAE) v5.0.

The EORTC QLQ-30 contains 30 questions that incorporate 5 functional scales (physical functioning, role functioning, emotional functioning, cognitive functioning, and social functioning), 1 global health status scale, 3 symptom scales(fatigue, nausea and vomiting, and pain), and 6 single items (dyspnea, insomnia, appetite loss, constipation, diarrhea, and financial difficulties). The participant answers questions about their health during the past week. There are 28 questions answered on a 4-point scale where 1 =Not at all (best) to 4 =Very Much (worst) and 2 questions answered on a 7-point scale where 1 =Very poor (worst) to 7 =Excellent (best). Higher scores in GHS and functional scales indicate better quality of life.

The symptom burden and physical condition will be measured by QLQ-CLL17. EORTC QLQ-CLL17 comprises 17 items grouped into 3 multi-item scales: 1) symptom burden, 2) physical condition/fatigue, and 3) worries/fears about health and functioning. Each question is rated using a 4-point response scale ("not at all," "a little," "quite a bit," and "very much") and the recall period for all items is the past 7 days. The scores of each subscale are calculated based on the EORTC's protocol and then transformed to a 0 to 100 scale. Higher scores represent higher levels of symptom burden, physical condition/fatigue, or worries/fears about health and functioning.