What controls the icons displayed on custom Multichannel activities on the Account timeline?
Users can view an additional icon and/or color to sub-categorize the custom Multichannel Activities. The icon type and color displayed on the activity in the Account timeline is controlled by the picklist value selected in the Icon_vod and Color_vod fields of the Multichannel_Activity_vod record.
The following is the list of icon types displayed for each possible Icon_vod picklist value:
CRM Help Documentation: Multichannel Activities on the Account Timeline