US Treasuries use actual/actual, corporate and municipal bonds usually use 30/360 which pretends every month has 30 days, and money-market instruments such as commercial-paper and repo use actual/360.
It sounds like plumbing, but actual/360 quietly pays more than it looks: a 5% rate on actual/360 earns 5% x 365 / 360 = 5.07% over a real year. On large notionals that gap is real money.
Example: $10,000,000 lent for 91 days at 5%. On actual/360 the interest is $10m x 0.05 x 91 / 360 = $126,388.89. On actual/365 it would be $124,657.53, a difference of $1,731.
Related: accrued-interest