Many View elements use the Id attribute to indicate the unique Id of the element.  Id values may not contain spaces.  Brick River mostly follows the CamelCase convention for Id values.

<Category Id="Category" Default="News Article" Invisible="false" />
<FieldSet Id="ContactFields" Cmd="Remove" />
<Relationship Id="Authors" Name="Authors" TableId="Contacts" ViewId="Authors" Cmd="After:Summary"  /> 

Id values must be unique within the parent element however they need not be unique if nested within different parents.  In the example below, it is valid to have a Id="FirstName" and Id="LastName" repeated for columns which are nested within the <Relationship> tag.
    <File Id="PrimaryImage" Multiple="false" Name="Primary Image"/>
    <Category Id="Type" CatType="Contact Types" Multiple="true"/>
    <FieldSet Id="NameFields" Name="Name Fields" Important="true">
        <Column Id="Name" Name="Full Name" Type="VarChar" Length="255" Important="true"/>
        <Column Id="FirstName" Name="First Name" Type="VarChar" Length="255" Important="true"/>
        <Column Id="LastName" Name="Last Name" Type="VarChar" Length="255" Important="true"/>
    <FieldSet Id="CreateUpdateFields" Name="Created/Last Updated">
        <Relationship Id="CreatedBy" Name="Created By" TableId="Users" ReadOnly="true" NoOverride="ReadOnly" Multiple="false" DedupIgnore="true">
                <File Id="Photo" Multiple="false" Name="User Photo"/>
                <Column Id="FirstName" Name="First Name" ColumnName="First_Name" Type="VarChar" Length="255"/>
                <Column Id="LastName" Name="Last Name" ColumnName="Last_Name" Type="VarChar" Length="255"/>