In fact, the Guard doesn't 'create' or 'sanction' settlement charters. In contrast, they do respond to requests for support in establishing settlements, refurbishing settlements, rebuilding, renovating, restoring, restocking, resettling, etc. The Guard want to help common mice get on surviving and thriving.
A settlement charter / constitution / deed / etc. is a documented statement of cultural and/or ethical binding ties that a group of settlement mice are willing to be governed by or are intending to govern by.
In the case of Grasslake and Shaleburrow these are rather democratic and state how elections are handled, how taxation is handled, what citizenship requires or means, and other stuff of that nature.
In Elmmoss and Elmwood, Dawnrock, and Ironwood there are monarchies with a sort of constitutional declaration of how the monarchy claims legitimacy, how rulership is passed, what rights belong to non-royals or non-nobles, and stuff about taxation, representation, grievances, etc.
In Lillygrove, Rustleaf, Pt Sumac, Darkwater, Shorestone and Barkstone there is something more like a republic council made up of the separate groups of mice; such as the artisan classes in Barkstone, Shorestone, and Rustleaf, or the regions of the city in Lillygrove and Darkwater, or the leading merchants in Pt Sumac. The council represents specific constituents (sometimes in corrupt ways) to their own conscience attempting to better their lives and balance the cries of other council members. Some councils have a top-level leadership like Pt Sumac, while others are a gridlock of equal-power-players like Barkstone and Shorestone. Lillygrove has layered leadership, but no single top-mouse leader.
In Mapleharbor, Lockhaven, Ivydale, and several others, there is a sort of mayoral model of democracy, wherein there is a clear top-mouse leader and certain advisory layers whether elected or appointed that are assigned to be representatives of the mice. It doesn't mean the individual mice cannot speak for themselves, such as a protest or riot, but it offers a political career path for a middle-layer engagement of civic duties.
In Wolfepointe, Wildseed, Sandmason, and several others, there is something like a tribal leadership pattern. It kinda mixes elements of monarchal and republic governance, and includes from deocratic features. It can be chaotic, but it can also be stable. Lonepine is another example of a tribal model, but has advanced beyond tribes of kin into tribes of spiritual beliefs. In Sandmason, the tribes are mostly formed by kinship, but also by affiliation of artisanship or slavery such that the clans are quite large--they're progressing toward a model more like Lillygrove. Wolfepointe is primitive tribalism in which the kinship holds sway, and something of a household clan matters, but as soon as a mosue reaches adulthood, htey could potentially declare themselves the head of a new clan and take up a role in civic leadership based soley upon proving themselves among other clan leaders.
Lastly, the recent upheaval in Flintrust replaced a rather mayoral model with another mayoral model. The difference is that the top-layer is a council, the middle-layer is a council, and the lowest layers are nigh unto slavery. Previously they had more autonomy and liberty, but now they have bread and circuses which they didn't previously have. It's a work in progress; it is kinda progressive, but needs to root out corruption, stabilize logistics, and entrench empathy among the governance layers.