All static contents must be positioned before all flows inside a page sequence.
For example, this XSL-FO will generate a strict formatting error, because the second static content is positioned after the flow:
<?xml version="1.0" encoding="utf-8"?>
<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
<fo:layout-master-set>
<fo:simple-page-master master-name="all-pages" page-width="8.5in" page-height="11in">
<fo:region-body padding="6pt" margin="0.7in" region-name="xsl-region-body" column-gap="0.25in"/>
<fo:region-before padding="6pt" region-name="xsl-region-before" display-align="after" extent="0.7in"/>
<fo:region-after padding="6pt" region-name="xsl-region-after" display-align="before" extent="0.7in"/>
</fo:simple-page-master>
<fo:page-sequence-master master-name="default-sequence">
<fo:repeatable-page-master-reference master-reference="all-pages"/>
</fo:page-sequence-master>
</fo:layout-master-set>
<fo:page-sequence master-reference="default-sequence">
<fo:static-content flow-name="xsl-region-before" font-size="12pt" font-family="Times New Roman">
<fo:block>sample header</fo:block>
</fo:static-content>
<fo:flow flow-name="xsl-region-body" font-size="12pt" font-family="Times New Roman">
<fo:block>sample text</fo:block>
</fo:flow>
<fo:static-content flow-name="xsl-region-after" font-size="12pt" font-family="Times New Roman">
<fo:block>sample footer</fo:block>
</fo:static-content>
</fo:page-sequence>
</fo:root>
The following XSL-FO will render fine:
<?xml version="1.0" encoding="utf-8"?>
<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
<fo:layout-master-set>
<fo:simple-page-master master-name="all-pages" page-width="8.5in" page-height="11in">
<fo:region-body padding="6pt" margin="0.7in" region-name="xsl-region-body" column-gap="0.25in"/>
<fo:region-before padding="6pt" region-name="xsl-region-before" display-align="after" extent="0.7in"/>
<fo:region-after padding="6pt" region-name="xsl-region-after" display-align="before" extent="0.7in"/>
</fo:simple-page-master>
<fo:page-sequence-master master-name="default-sequence">
<fo:repeatable-page-master-reference master-reference="all-pages"/>
</fo:page-sequence-master>
</fo:layout-master-set>
<fo:page-sequence master-reference="default-sequence">
<fo:static-content flow-name="xsl-region-before" font-size="12pt" font-family="Times New Roman">
<fo:block>sample header</fo:block>
</fo:static-content>
<fo:static-content flow-name="xsl-region-after" font-size="12pt" font-family="Times New Roman">
<fo:block>sample footer</fo:block>
</fo:static-content>
<fo:flow flow-name="xsl-region-body" font-size="12pt" font-family="Times New Roman">
<fo:block>sample text</fo:block>
</fo:flow>
</fo:page-sequence>
</fo:root>