Basic Direct Access Method (BDAM) arranges records in any sequence your program indicates, and retrieves records by actual or relative address. If you do not know the exact location of a record, you can specify a point in the data set where a search for the record is to begin. Data sets organized this way are called direct data sets.
BDAM is no longer supported by COBOL. however as stated earlier, BDAM allows retrieval of data by relative block number. With BDAM, you have a method to generate the block number and that block contains the data record. There is no key required, you can read the one block of data you need, but the only way to sequentially access the blocks is one by one.
It’s function, for the most part, has been replaced by RRDS VSAM files. Some vendors still use it. Unless ‘am mistaken, Panvalet files are BDAM. Also, I think IAM files, although simulating VSAM, are actually BDAM files.