here is what sponsorship undertaking means provided for:
Applications from parents, grandparents, and other dependant relatives under Rule 317 of the Immigration Rules must be accompanied by a signed understaking from the sponsor. The sponsorship undertaking is available online in the family members guidance (INF 6) on the ukwebsite or from your visa application centre.
Do you guys think fiancee and spouse visa need one? i didn't submit any which i thought i am not under into that catergory and none on the UKVACS ask me to provide one which she even inspect all my documents one by one...please enlighten me with this.