The inner layer is mainly used for extremely cold conditions and longer term exposure. In a similar fashion to the base layer, its main job is to retain body heat and wick moisture away from the base layer. This layer should be snug but not too constricting.
The mid layer is another level of insulation that retains heat and transfers moisture away from the body. Wool, synthetic fibres or down make excellent mid layers. Fleece fabric requires less care than wool and down is an excellent insulator as long as it remains dry. All these fibres work on the same principle of trapping small pockets of warm air amongst the fibres. Synthetics outperform wool in terms of moisture control because they absorb very little water. Wool can absorb up to 30% of its own weight in water. So wool is not generally recommended for highly active sports.
A number of things need to be borne in mind when choosing a mid layer. For example, venting by way of zips for flexibility of internal temperature and properly fitting size. Pullover garments are an excellent choice with baggier sleeves to allow them to be rolled up. Zip necked T-shirts allow for good ventilation and keep the neck warm when zipped up. Again, the key is to have thin layers of air trapped between garments and not too much excess that will cause bunching, add weight or restrict movement. Too much space will push air out when moving and too large an airspace will be difficult for the body to warm up. Snug but not constricting is the key to all layers.