Here's the rule for player eligiblity on Yahoo...
Yahoo.com wrote:How is position eligibility established?
At the beginning of the Fantasy Baseball season, Yahoo! Sports placed all players into positions based in part on information from official rosters, but focusing mainly on past performance. Some players appear in multiple position lists and are eligible to fill any of those positions.
Please note the following conditions that apply to a player's position eligibility.
1. A player's position eligibility will not be adjusted prior to the beginning of the season. (Alex Rodriguez, Jose Reyes and all other players beginning the season at a new position are required to meet the requirements outlined below.)
2. Players will not lose eligibility at a previously established position at any time. (For example, Alex Rodriguez will retain his eligibility at the SS position for the 2004 season.)
3. It is not possible to customize this setting within Custom Leagues. All leagues are subject to the same constraints.
Gaining eligibility at a new position:
If a position player makes 15 appearances at a new position during this season, he will become eligible to play that position in Yahoo! Sports Fantasy Baseball.
Pitchers need to make three starts to become eligible as a starter and five relief appearances to qualify as a reliever.
To find the number of games in which a player has appeared at a certain position, click on their name. You will be taken to their Player Profile. From there, click on the "Career" link. You will be able to view the player's statistics for the season, and the number of games played at each position.
Players will not necessarily be eligible for all positions listed in their Player Profile.
Appears to be appearances
If anyone doesn't/didn't know you can check a players appearances on a Player's page under situational stats.