Refer to figure.
SHELL MODEL

The SHELL model is a tool that shows the interaction that takes place between its components. The central 'L' (red square of the figure) of the model refers to 'Liveware' which is the pilot. The pilot is at the center of the model because he/she interacts with the rest of the components around him/her.

'H' on top of the model refers to 'Hardware'. The Liveware-Hardware interaction (L-H interaction) refers to the interaction between the pilot and the hardware (cockpit layout, physical aircraft structure, instruments presentation and position of controls). For example, a critical instrument located outside the field of view of the pilot will make it hard for him/her to cross check it.

'S' on the left side of the model refers to 'Software'. The Liveware-Software interaction (L-S interaction) refers to the interaction between the pilot and the software (procedures, training, support, manuals, checklist layouts, maps and charts, computer programmes). For example, if the charts are expired the pilots may use incorrect data.

'E' on the right side of the model refers to 'Environment'. The Liveware-Environment interaction (L-E interaction) refers to the interaction between the pilot and the environment (temperature, ambient light, noise, vibration, humidity). For example, in an environment of very high temperature the pilot's performance will be degraded.

'L' at the bottom of the model refers to 'Liveware'. The Liveware-Liveware interaction (L-L interaction) refers to the interaction between the pilot and the rest of the crew. For example, an autocratic captain may cause stress to the rest of the crew.

L → Liveware
S → Software
H → Hardware
E → Environment