minor edits for data_levels section.

data_model/data_levels.md

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-# Data Standard Levels
+# Data Levels and Clinical Data Tiers
 
-The HTAN data model divides assay data into levels. Each assay type has levels progressing from raw data to more processed data.  This is illustrated in the following figure for sequencing data.  Please see each assay's page within [Data Standards](https://humantumoratlas.org/standards) for more information.
+The [Data Standards](https://humantumoratlas.org/standards) pages of the HTAN Data Portal provide a detailed overview of the HTAN Data Model. A general overview of how the HTAN Data Model is organized is provided in the next sections.
+
+## Assay Data Levels
+The HTAN Data Model divides assay data into levels. Each assay type has levels progressing from raw data to more processed data.  This is illustrated in the following figure for sequencing data.  Please see each assay's page within [Data Standards](https://humantumoratlas.org/standards) for more information.
 
 ![HTAN Sequencing Data levels](../img/sequencing_data_levels.svg)
 
-Clinical data is organized into tiers.  These tiers are described more on the [Clinical Data Page](https://humantumoratlas.org/standard/clinical). 
+For Sequencing data, the HTAN Model Data levels help distinguish data as Open Access versus Controlled Access.  For example, level 1 and 2 sequencing data are Controlled Access data.     
+
+## Clinical Data Tiers
+Clinical data is organized into three tiers.  Tier one is based on the NCI's Genomic Data Commons (GDC) clinical data model.  Tiers two and three are disease-agnostic (tier 2) and disease-specific (tier 3) extensions to the GDC model. These tiers are described more on the [Clinical Data Page](https://humantumoratlas.org/standard/clinical). 
 
-The [Data Standards](https://humantumoratlas.org/standards) provides up-to-date details regarding the HTAN Data Model. For additional notes regarding how to submit data for each data type, please see the [Specific Assay/Data Element Details](../data_submission/specific_details.md) page of this manual.
+## Specific Details
+Please see [Data Standards](https://humantumoratlas.org/standards) for up-to-date details regarding the HTAN Data Model, including attribute descriptions and requirements. For additional notes regarding how to submit data for each data type, please see the [Specific Assay/Data Element Details](../data_submission/specific_details.md) page of this manual.