Charge modifiers change the appearance of the charge in some respect. There are hundreds of these, most of which apply only to one particular charge, or to a particular type of charge. I have not listed them as I don't think it is particularly helpful. Almost all of them will be recognised, but not many of them will actually be drawn.
There are however some common modifiers that are mostly implemented, so I have listed these.
Common Prefix Modifiers
The word demi can be used before any charge to indicate that only half of it should be shown. This is usually the top half of animals and the sinister half of inanimate objects.
Other common prefixes that apply to a variety of charges include:
heads of, dexter, sinister, branch of, leaf of, a pair of, a bundle of
Common Postfix Modifiers
The term en soleil after a charge causes that charge to be drawn with yellow sun rays surrounding it.
A limited number of charges can be fimbriated of a different colour, in which case they will be drawn with a narrow border or outline in that colour. Only charges that comprise that simple, geometric shapes can be fimbriated1, an error message will be generated for charges that do not fall into this group.
1 This is an implementation limitation rather than a heraldic one - it is implemented by doubling the width of the SVG "stroke" and assigning the required colour to that stroke, hence it only works for those charges that have a single, continuous border.
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