The UPC does not have an "official stance" but what we call the three-step plan of salvation (repentance, water baptism, and Spirit baptism) is believed by many and is often presented as the "official stance." This three-step plan of salvation should not be confused with another three separate experiences preached by some.
When the Holy Spirit was poured out in the late 1800's and early 1900's, those who received the experience were Christians seeking something more from God. Some were from "holiness" type churches and some were not. Those from "holiness" churches already believed in a post-salvation experience called "sanctification" or "the second blessing" so to them the Holy Ghost Baptism was a separate and subsequent experience --a third experience. The would testify. "I'm saved, sanctified, and filled with the Holy Ghost."
In the early 1900's a preacher named Durham from Chicago preached a message all over the place called "The Finished Work of Calvary." He taught that sanctification or holiness was not a separate experience from salvation but that when we got saved we were made holy and set apart for the Lord and we are to grow in that holiness/sanctification for the rest of our lives. Those from the "holiness" groups that did not accept the finished work doctrine still maintained that sanctification is a separate work and today they are the Church of God (Cleveland, TN), other Church of God groups which split off the Cleveland, TN group, Pentecostal Holiness, and Church of God in Christ. Those that preached the finished work doctrine later organized into what is now the Assemblies of God.
Around 1913 another teaching became popular which was called the new issue. This teaching said that the early church baptized using the name of Jesus (with or without the titles Lord and Christ) instead of the traditional baptism which was being used in just about every church or denomination in the world. Some taught that the name of the Father is Lord; the name of the Son is Jesus; and the name of the Holy Ghost is Christ so they proposed a triune name for a triune God and baptized that way. Others taught that the single name Jesus is the name of the Father and of the Son and of the Holy Ghost. Many people were re-baptized using these various formulas but re-baptism was not considered salvational --just a step back into the first century teaching of the Apostles.
Later, some began to teach that water baptism in Jesus' name (with or without the titles Lord and Christ) was necessary for salvation. Some began to teach that water baptism was the birth of water spoken of in
John 3:5.
When the UPC was formed in 1945, two organizations came together and merged. One group, the PCI (Pentecostal Church Inc) was predominantly made up of ministers who believed that a person was saved prior to water and Spirit baptism. The other group, the PAJC (Pentecostal Assemblies of Jesus Christ) was predominantly made up of ministers that believed a person was not saved until completing the three steps of repentance, water baptism, and Spirit baptism. So that the two groups could merge a "fundamental doctrine" statement was written up that was ambiguous enough that both groups could agree with.
Since 1945 those who believe in a three-step plan of salvation have pretty well pushed out the one-steppers (or intimidated them into silence) so now many in the UPC preach and teach that a person is not saved until they have completed the three steps of repentance, water baptism, and Spirit baptism.