Using attributes and elements and text

I have never see a conclusive definition on when using attributes
instead of elements (besides one that says by using attributes one is
sure that it is a leaf of the tree), and also using text values in
nodes uith childs. Let me put it this way; what are the functional
diferences between these constructs?

<XHoras>
        <Assuntos>
                <Assunto K="123">
                        As armas e os barues assinalados
                        <Detalhes>
                                <Tipo>0</Tipo>
                                <Detalhe/>
                        </Detalhes>
                </Assunto>
        </Assuntos>
</XHoras>

<XHoras>
        <Assuntos>
                <Assunto>
                        <K>123</K>
                        As armas e os barues assinalados
                        <Detalhes>
                                <Tipo>0</Tipo>
                                <Detalhe/>
                        </Detalhes>
                </Assunto>
        </Assuntos>
</XHoras>

<XHoras>
        <Assuntos>
                <Assunto>
                        <K>123</K>
                         <Desc>
                             As armas e os barues assinalados
                         </Desc>
                        <Detalhes>
                                <Tipo>0</Tipo>
                                <Detalhe/>
                        </Detalhes>
                </Assunto>
        </Assuntos>
</XHoras>