The INVEST mnemonic for Agile software development projects was created by Bill Wake[1] as a reminder of the characteristics of a good quality Product Backlog Item (commonly written in user story format, but not required to be) or PBI for short.
One of the characteristics of Agile Methodologies such as Scrum, Kanban or XP is the ability to move PBIs around, taking into account, amongst other criteria, their business value.
According to Agile methodology, while the PBI lies in the product backlog it can be rewritten or even discarded, depending on business, market, technical or any other type of requirement by team members.
A draft story should not form a rigid contract detailing the design but should instead talk of business outcomes the user will be able to conduct once implemented.
The wording of Bill Wake's original article speaks to this negotiation process, "Over time, the card may acquire notes, test ideas, and so on, but we don’t need these to prioritize or schedule stories."
[3] Bill Wake has expressed that were he to re-pick INVEST today, he would remove "Estimability" and utilize the "E" to instead emphasize an aspect of the "V for Valuable" criteria.
Allen Holub has suggested that the British English meaning should be embraced, seeing the giving of an estimate as being harmful to the software development process.