What best interests of the child means
Best interests of the child is the guiding standard behind nearly every custody decision in the United States. Rather than treating custody as a prize to be split, courts ask what arrangement best serves the child. What that means in practice is defined by each state, sometimes as a long statutory checklist and sometimes through case law, so the exact factors a judge must weigh vary widely.
Common threads run through most versions: each parent's history of caregiving, the stability of each home, each parent's willingness to support the child's relationship with the other parent, the child's needs, and safety concerns including any history of abuse. Many of these are pattern questions, answered by what happened repeatedly over time rather than by a single event.
This is why documentation aligns so naturally with the standard. A steady record of school involvement, medical appointments, honored parenting time, and cooperative communication is direct evidence on the factors a judge actually scores.
Example: a factor asking which parent better supports the child's bond with the other parent is answered persuasively by a log showing you offered makeup time and kept the other parent informed, not by assertions alone.