Not all archives look alike, collect alike, or organize themselves alike. Paying attention to the sometimes subtle differences will greatly help you understand what you can expect to find in any collection.
Archives generally seek to collect materials related to, originating in, or produced by a certain time period, social group, geographical location, or even a specific individual.
Depending on the focus, there will be differing types of survivable materials to collect. The collection you encounter is not the complete picture. Instead, it is a grouping of related materials that have survived, were preserve, collected, organized, and brought together into a single entity for some purpose.
Most of these aspects will be described or listed out by the archive owners/organizers. Always look for a mission statement, project description, collection focus, or any other language that will help you better understand the intention of the archive. This will typically yield information on its limitations (especially if self-imposed).
Look through the image to the left. While this is in no ways an exhaustive depiction of how we get from "there" to "the archive", it does illustrate some fundamentally important realities.
1. We don't have everything! (no surprise?)
2. There are plenty of places for bias and power/authority to determine what gets recorded and what gets preserved.
3. Authorship and audience can be crucial in survivability.
4. The digital collection might be a much smaller collection than its physical collection "parent".