They typically only become an issue in hotly contested proceedings, when spouses can't agree on custody terms and their inability to do so forces the court to decide. (info.legalzoom.com)
However, the process of moving from separation to divorce usually does not, by itself, change custody terms. (info.legalzoom.com)
When de facto refers to divorce, it is the arrangement that's in place at the time parents file, before any court order yet exists to establish custody terms or visitation. (info.legalzoom.com)