Many scientific journals, such as Chemical Reviews, require the table of contents and headings in an article follow a decimal outline format. The consensus among scientists is that the decimal outline is more accurate, more concise, and more contemporary than the alphanumeric outline. The format is helpful to readers. It allows them to glance at the table of contents and follow the logic behind the work being presented. It is helpful to researchers as well. It serves to organize work for presentation and reveals gaps in logic or extraneous material. In short, an outline created before you finish your document prompts you to consider what your audience needs to know to understand and validate your work.
The numerical outline does not use complete sentences because the headings are used to label each section within the document and are also used to create the Table of Contents.

Create a Hierarchy of Ideas and Data

The numerical outline may have an infinite number of levels. For the purpose of illustration, this handout considers three levels. The first level is the main topic being discussed in that section. Think of a section as composed of at least one paragraph. If there are multiple paragraphs in a section, each should relate to this main topic. It is unnecessary to go past the first level if this level needs no further elaboration, but it is possible to go deeper and is often needed. Levels can keep going deeper, until a topic is exhausted and a new first heading is required.
Ex. 1. Introduction to gyroscopes
The next level, the sub-section, is indicated by the same section number, followed by a decimal point and a number indicating its relationship to the top level. It is indented as another way to show this relationship. 
Ex.4. Selected background molecules
   4.1. Molecular turnstiles
           4.2. Species with N-ML2-N rotators where N denotes a disubstituted pyridine ligand
           4.3. Other stators in which two bridges connect the termini of the rotatorThe next lowest level, the sub-sub-section, is also indicated by the same main section number, then a decimal point and the sub-section number, followed by a decimal point and a number indicating its relationship to the previous level. It is also indented to show this relationship.
 
Ex. 6. Systems from the Gladysz group
           6.1. Syntheses involving olefin metathesis
           6.2. Reactions that alter the coordination number
           6.3. Substitution reactions
                 6.3.1. Replacements of chloride ligands
                 6.3.2. Replacements of carbonyl ligands
           6.4. Dynamic properties: general concepts
           6.5. Measured or bounded rotational barriers
           6.6. Key solid state properties
           6.7. Towards molecular gyroscope