[NFe] Nota fiscal eletrônica: significado da tag <mod>

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<NFe xmlns="http://www.portalfiscal.inf.br/nfe">
<infNFe Id="NFe43120103987283000133550010000006241607910981" versao="2.00">
   <ide>
     <cUF>43</cUF>
     <cNF>60791098</cNF>
     <natOp>5.101 - VENDA</natOp>
     <indPag>1</indPag>
     <mod>55</mod>
     <serie>1</serie>
     <nNF>624</nNF>
     <dEmi>2012-01-25</dEmi>
     <tpNF>1</tpNF>
     <cMunFG>4313409</cMunFG>
     <tpImp>1</tpImp>
     <tpEmis>1</tpEmis>
     <cDV>1</cDV>
     <tpAmb>2</tpAmb>
     <finNFe>1</finNFe>
     <procEmi>0</procEmi>
     <verProc>2.02</verProc>
   </ide>
   <emit>
     <CNPJ>03987283000133</CNPJ>
     <xNome>MINHA EMPRESA TESTE LTDA.</xNome>
     <xFant>MINHA EMPRESA FANTASIA TESTE</xFant>
     <enderEmit>
        <xLgr>RUA TESTE</xLgr>
        <nro>123</nro>
        <xBairro>CENTRO</xBairro>
        <cMun>4312345</cMun>
        <xMun>PORTO ALEGRE</xMun>
        <UF>RS</UF>
        <CEP>93000000</CEP>
        <cPais>1058</cPais>
        <xPais>BRASIL</xPais>
        <fone>5111223344</fone>
     </enderEmit>
     <IE>0868689965</IE>
     <CRT>1</CRT>
  </emit>
  <dest>
     <CNPJ>99999999000191</CNPJ>
     <xNome>NF-E EMITIDA EM AMBIENTE DE HOMOLOGACAO - SEM VALOR FISCAL</xNome>
     <enderDest>
        <xLgr>AV. DOS TESTES</xLgr>
        <nro>202</nro>
        <xBairro>CENTRO</xBairro>
        <cMun>4369854</cMun>
        <xMun>PORTO ALEGRE</xMun>
        <UF>RS</UF>
        <CEP>93000000</CEP>
        <cPais>1058</cPais>
        <xPais>BRASIL</xPais>
     </enderDest>
     <IE>ISENTO</IE>
  </dest>
  <det nItem="1">
     <prod>
        <cProd>15036</cProd>
        <cEAN/>
        <xProd>PRODUTO TESTE REF 123123</xProd>
        <NCM>84123123</NCM>
        <CFOP>5101</CFOP>
        <uCom>UN</uCom>
        <qCom>1.0000</qCom>
        <vUnCom>590.0000</vUnCom>
        <vProd>590.00</vProd>
        <cEANTrib/>
        <uTrib>UN</uTrib>
        <qTrib>1.0000</qTrib>
        <vUnTrib>590.0000</vUnTrib>
        <indTot>1</indTot>
     </prod>
         <imposto>
            <ICMS>
               <ICMSSN101>
                  <orig>0</orig>
                  <CSOSN>101</CSOSN>
                  <pCredSN>2.82</pCredSN>
                  <vCredICMSSN>16.64</vCredICMSSN>
               </ICMSSN101>
            </ICMS>
            <PIS>
               <PISOutr>
                  <CST>99</CST>
                  <vBC>0.00</vBC>
                  <pPIS>0.00</pPIS>
                  <vPIS>0.00</vPIS>
               </PISOutr>
            </PIS>
            <COFINS>
               <COFINSOutr>
                  <CST>99</CST>
                  <vBC>0.00</vBC>
                  <pCOFINS>0.00</pCOFINS>
                  <vCOFINS>0.00</vCOFINS>
               </COFINSOutr>
            </COFINS>
         </imposto>
      </det>
      <total>
         <ICMSTot>
            <vBC>0.00</vBC>
            <vICMS>0.00</vICMS>
            <vBCST>0.00</vBCST>
            <vST>0.00</vST>
            <vProd>590.00</vProd>
            <vFrete>0.00</vFrete>
            <vSeg>0.00</vSeg>
            <vDesc>0.00</vDesc>
            <vII>0.00</vII>
            <vIPI>0.00</vIPI>
            <vPIS>0.00</vPIS>
            <vCOFINS>0.00</vCOFINS>
            <vOutro>0.00</vOutro>
            <vNF>590.00</vNF>
         </ICMSTot>
      </total>
      <transp>
         <modFrete>1</modFrete>
      </transp>
      <cobr>
         <dup>
            <nDup>624</nDup>
            <dVenc>2013-02-08</dVenc>
            <vDup>590.00</vDup>
         </dup>
      </cobr>
      <infAdic>
         <infCpl>TESTE DE INFORMACOES ADICIONAIS</infCpl>
      </infAdic>
   </infNFe>
</NFe>