This defines the purpose of the certificate and whether it can be used as client certificate. The following is a sample of an expected output from the command:

openssl x509 -in your_cert.filename -purpose

Certificate purposes:
SSL client : Yes
SSL client CA : No
SSL server : Yes
SSL server CA : No
Netscape SSL server : Yes
Netscape SSL server CA : No
S/MIME signing : No
S/MIME signing CA : No
S/MIME encryption : No
S/MIME encryption CA : No
CRL signing : Yes
CRL signing CA : No
Any Purpose : Yes
Any Purpose CA : Yes
OCSP helper : Yes
OCSP helper CA : No

Please ensure that the certificate you purchase has "YES" for SSL client. As noted, this certificate can be used for both server and client purposes.