loop.html

<!-- the standard loop for all your posts -->
{% for post in site.posts %}

    <!-- check if there is a portfolio thing set inside the Front Matter 
        Note: Do not set "portfolio: false" on normal posts - this is not needed
        and could lead to problems
    -->
    {% if post.portfolio %}
        <!-- do nothing here -->
    {% endif %}
    
    <!-- then display all the normal posts -->
<article class="post">
    <h2>{{ post.titl }}</h2>
    {{ post.excerpt }}
</aticle>
{% endfor %}

not-portfolio-post.md

title	layout
I'm not a Portfolio Item!	post

Lorem Ipsum Dolor

Just do not write "portfolio" inside the Front Matter. That's it.

portfolio-post.md

title	layout	portfolio
I'm a Portfolio Item!	portfolio	true

Lorem Ipsum Dolor

portfolio.html

{% for post in site.posts %}
<!-- again we loop through the posts, because technically the Portfolio Stuff is a post -->

  <!-- this time we only want posts where portfolio is true (or set at all) --> 
  {% if post.portfolio %}
    <article class="portfolio--article">
      <!-- the title-->
        <h2>{{ post.title }}</h2>
      
        <!-- the content -->
      <p>{{ post.content }}</p>
    </article>
  {% endif%} <!-- end of every post that has portfolio: true -->

{% endfor %} <!-- end of the loop -->