On average, 10%-30% of a resource's time is spent on administration activities. To that, add time-off and general non-project work, and it soon becomes clear that a resource typically has much less availability than assumed for projects.
To handle resource overhead (e.g., administration, time-off, etc.) and non-project work (e.g., base services, general support or consulting, etc.) in ResourceFirst, there are several options, as follows:
Option 1: Ignore (and, optionally, track overhead actuals only)
This involves simply not forecasting this type of activity, though Overhead categories can be defined in the Administration center that people entering timesheets can post time against.
Cons: Forecast may not match Actuals, and resource availability may be overstated.
Option 2: Create Bucket Project(s)
This option involves creating a "bucket" project (typically yearly) with each type of overhead or non-project work as a phase or task. For instance, a project called 2018 Overhead for Dept XYZ could include line items for Administration, Time-Off, General Consulting, Support, and more. Resources can be partially assigned to those items by percentage (using task scheduling) across the whole year, and can enter time against those tasks as needed.
Pros: Allows flexibility in forecasting different kinds of administrative and/or non-project work (meetings, time off, etc.)
Cons: Maintaining list is additional overhead
Option 3: Lower Capacity
Pros: Adequately represents how much time is available
Cons: Headcount reports may be confusing
Recommendation: A Hybrid Approach
A best practice is to use option 1 for general overhead items (not forecasting them, but optionally allowing time-tracking of admin work and time-off via defined Overhead categories) and option 2 (bucket projects) for other non-project work, such as support, operational work, etc. This still maintains a reasonable forecast while discretely accounting for heavier allocations such as base services and support.
See also, What is Overhead Data and Where Does it Come From? and When Should Overhead be Used and When Should These Items Be Treated as Projects?