The primary bud is the most prominent in the group and will be the bud that breaks opens and grows during the spring flush of growth. Inside the bud is a vegetative shoot flanked on each side by a set of catkins (male flowers). If all goes well, the primary bud grows and the smaller buds below it will remain dormant.
However, the tree creates secondary, tertiary and quaternary buds just in case something happens to injury or destroy the primary bud's new growth. A late spring frost, a freak hail storm, or even an outbreak of sawflies can destroy the tree's first attempts at new spring growth. That's when the secondary bud comes into play. The secondary bud will start growth in response to the lost of the primary bud. We saw this happen just last year in response to a late spring freeze.
Back in 2007, I was really glad that pecan trees have a built in back-up plan for restarting growth following the loss of new growing points. We established a new orchard using container grown trees in the fall of 2006. On Easter weekend of 2007, we experienced bone chilling cold that froze all the emerging green tissues off the young trees. A couple of weeks later, secondary buds broke and the trees resumed building new shoots and leaves. Later that summer, the Neosho river spilled over its banks and we experienced the second highest flood on record. The flood waters were so deep that they completely covered the newly established trees and killed all green tissues. Once the flood receded, tertiary and quaternary buds broke and young trees struggled back to life.