When you’re planning within our tool, we assume that you might start with a high-level idea as to how a person might be staffed. Usually this is a guess (example: you create an assignment for 50% for 6 weeks on Project A). Really at this stage of planning, you are just reserving the team member on the project for that time frame. As you move closer to the project start date, you will likely refine the 50% assignment into more realistic chunks of time (10%, 20%, 50% for days or weeks during that window of time).
You can split up that long 50% assignment on the schedule. To do this, simply click on the assignment where you want to segment and select the % Allocation. Click the "Split between X dates" to divide into sections. Now, you can change the allocation for one section of the assignment to refine your estimate.