diff --git a/spec/docs/logicalSource.md b/spec/docs/logicalSource.md
index 3e387a6..627db67 100644
--- a/spec/docs/logicalSource.md
+++ b/spec/docs/logicalSource.md
@@ -11,5 +11,63 @@ A <dfn data-lt="iteration">logical iteration</dfn> is an item in the sequence pr
 A <dfn>data source</dfn> is an abstract concept that represents a source of data that can be accessed via a [=logical source=]. A [=data source=] can be a file, a database, a web service, or any other source of data.
 
 <aside class="note">
-There can be many different types of [=reference formulation=]. The known types, and the details of how a reference formulation is handled and implemented for each data format, are specified in [[RML-IO-Registry]].
+There can be many different types of [=reference formulation=]. RML-Core covers CSV and JSONPath, RML-IO extends this further with XML and SQL relational databases. Other known types, and the details of how a reference formulation is handled and implemented for each data format, are specified in [[RML-IO-Registry]]. This way, implementations are not required to support all defined reference formulations to be compliant with RML-Core.
 </aside>
+
+## Reference Formulations
+
+RML-Core covers `rml:CSV` and `rml:JSONPath` as the minimum reference formulations to be supported by any implementation for referencing data in CSV and JSON documents.
+
+Each Logical Source has a reference formulation to define how to reference
+to elements of the data of the input source.
+Several reference formulations (`rml:ReferenceFormulation`)
+are defined in this specification:
+
+- `rml:CSV`: CSV documents
+- `rml:JSONPath`: JSON documents
+
+See [[RML-IO-Registry]] for a detailed specification of these reference formulations.
+
+## Access descriptions
+
+RML-Core requires the `rml:PathFile` access description to be supported by any implementation to access files with an absolute or relative path.
+This access description allows accessing files with relative and absolute paths from:
+
+- `rml:CurrentWorkingDirectory`: relative to the current working directory of the RML processor.
+- `rml:MappingDirectory`: relative to the location of the RML mapping.
+- A string Literal: a string describing an absolute path against which relative paths are resolved, similar to the Base URI in [RFC3986](https://www.rfc-editor.org/rfc/rfc3986).
+
+If `rml:root` is not specified, it defaults to `rml:CurrentWorkingDirectory`.
+
+| Property    | Domain                    | Range                                                              |
+| ----------- | ------------------------- | ------------------------------------------------------------------ |
+| `rml:root`  | `rml:FilePath`  | `rml:CurrentWorkingDirectory`, `rml:MappingDirectory` or `Literal` |
+| `rml:path`  | `rml:FilePath`  | `Literal`                                                          |
+
+Example of accessing a CSV file relative to the current working directory.
+The file's absolute path is `$CURRENT_WORKING_DIR/file.csv` where `$CURRENT_WORKING_DIR` is
+the location of the RML mapping.
+
+<pre class="ex-source">
+&lt;#RelativePathCWD&gt; a rml:LogicalSource;
+  rml:source [ a rml:FilePath;
+    rml:root rml:CurrentWorkingDirectory;
+    rml:path "./file.csv";
+  ];
+.
+</pre>
+
+Example of accessing a JSON file relative to the path of the mapping.
+The file's absolute path is `$MAPPING_DIR/file.json` where `$MAPPING_DIR` is
+the location of the RML mapping.
+
+<pre class="ex-source">
+&lt;#RelativePathMapping&gt; a rml:LogicalSource;
+  rml:source [ a rml:FilePath;
+    rml:root rml:MappingDirectory;
+    rml:path "./file.json";
+  ];
+  rml:referenceFormulation rml:JSONPath;
+  rml:iterator "$";
+.
+</pre>