If you install Win7 into unallocated free space on a HD the installer creates a 100 MB hidden partition (no drive letter) which contains the booting files. Win7 Ultimate can use the partition to run BitLocker. Win7 Professional and lesser versions don't have BitLocker so you scratch your head and wonder why you have the partition. 

If you install Win7 into an already created primary partition, a SRP is not created.

If you install Win7 into a logical volume inside an extended partition, a SRP is created in any unallocated space outside the extended partition.

If you install Win7 into a logical volume inside an extended partition and there is a primary partition outside the extended partition, the booting files are installed into the primary partition.

If you only have an extended partition on the HD with no primary partition or unallocated free space outside the extended partition, Win7 will not install.